DH sounds like a jerk. I wonder if that's why he found himself single with a toddler in the first place. Think long and hard before having any more kids with him. I could understand him saying "Aw, man, i wanted him to open it at the party! Next time let's do the gifts together, ok?" That's communicating. "You ruined the WHOLE day!! You can't come to the party now, you have to stay home!" is way over the top. 

That is one of the worst undermining stories I've heard on here. If my DH did THAT I'd be gone so quickly. Think about this reaction (and his spending habits).

A good and well needed consequence for this situation is to separate your money and put him in charge of half the bills.  The extra that you suddenly have can go into an account to save up for a predictably needed lawyer.    Stop bailing him out and enabling his financial abuse of you.  

Yikes, MrsStepmother,

Your new husband sounds like one of the most difficult types of Disney Dads  encountered on this site; crying poor mouth when the bills come due yet managing to find sufficient funds to buy frivolous gits for their children. Hon, you’ve fallen for the ‘ol bait and switch from a guy that wants a ‘wife appliance’ who will pay the bills, clean the house, cook and warm his bed. Additionally, you’re expected to accept verbal abuse and never complain.

 I strongly advise you to ‘start as you mean to go on’ by refusing to accept the unacceptable from the beginning, rather than waiting until a pattern of tolerance has been established. Where do you see yourself and this relationship in the next year or two? Trust me when I say that men who behave so egregiously this soon in a marital relationship do not improve with age; things only worsen. Ask me how I know…

Both parties must discuss, the spending of the family.  No one should take it apond there self to spend $300 on a Birthday gift.   A must, car repair, home repair is different.  Then your SO being upset because you upset his Happy Family party.  This is not how it works.  
You have a DH problem.  Don't know if you are going to change him,  he need to see someone, 

Why do you tolerate him in your life?

Stop that!

Chef tried to buy his feral's love with a four wheeler which turned out to be nothing but a disaster! Oh and the scolding over perceived faux pas!

(((Shudder)))

So your husband went into automatic rage/punisher mode. Had a full on mantrum,

Yep, and the spending is off the charts.

Separate your finances now if you havent.

Make certain he is paying his fair shair of bills etc.

Start watching as more red flags crop up.

start researching for legal asistance to see what you might need to do for your future.

Stay vigilant on your BC so your not stuck.

"I told my DH that I will never pick out a birthday gift for SS again because everything I do for him turns into an issue and an argument."

Your entire post worries me for you, especially that last sentence.  First, DH has no reason to get mad at you for giving SS his gift early.  That's just silly.  Did he specifically say why that upset him?  It just seems petty, like he's looking for reasons to be an a&&hole to you.  And the effort to exclude you from going to the inlaws is utterly ridiculous and so incredibly childish - also, it's very hurtful toward you.  It almost seems like he's jealous of you trying to have a relationship with SS.  I post this a lot, and others probably get tired of seeing it, but my ex husband would do the same thing.  He would exclude me from things if I upset him regarding his son or anything really that he found offensive to his sensitive feewings.  Or, early on in my relationship with my ex, I might offer some sort of assistance (money or a ride, etc) to my ex SS when he was younger and it would just enrage my then-husband.  "He's not your son!  You don't get to decide or offer anything."  Things deteriorated over the years.  You should read my posts.

Also, the fact that he got such an elaborate gift for his 4 year old son leads me to believe that your DH wants to be the "cool parent."  This is not a good sign.

I urge you to open up a conversation wth DH about this incident.  Perhaps it will be an isolated thing.  But I can tell you that sometimes this type of immature behavior becomes a regular thing - and when that starts, it's time to jump ship. It can also be a sign of Disney parenting. 

Oh, and I also second what CLove said.  Keep your distance financially and know what your options are by speaking with an attorney - just in case "jump ship" is what you end up doing.  Definitely watch for red flag behaviors.  These issues often start out small and over the years become bigger and bigger, and the next thing you know, you've lost yourself and are miserable.

Please keep us posted and take care of yourself and your baby,

Reedle

Yeah I wouldn't be buying anymore gifts after DH's mantrum about your gift timing. 

So what if you give the gift now vs. later 

At least you spent your hard earned coins to even get him a gift. 

If you still want to do something nice for the SS out of the kidness of your heart, put some money in an UTMA, IUL, or something and give it to him when he's 18 

And why tf is your DH splurging on a $400+ gift for a 4year old then complaining about being broke? 

I'm always looking toward the future. What future do you hope for, and how will you get there? Do you think there is any chance that your DH will reign in his anger and his tactics to belittle you? If there is hope, how will he get there? His promise to "try harder," if he even does that, is not sustainable without professional guidance. Is he willing to see a therapist and DO THE WORK?

If he is, then maybe there is hope. If he's not, then what will YOU do to change this terrible situation for yourself and your son? You don't want your own child growing up thinking that this is a normal way to treat women.

If HE won't change, then it's up to you to make the changes that you need. The one thing you can't do is to make your partner change.

He is attached to BM still.  Hate and doing better than BM is his focus.  Not you or his new baby.  Let that sink in.  He is not truly available or free for a devoted relationship with you.  Did he rush into a new relationship after the breakup with BM? Did he spend time alone to shake off the marriage and find himself?   I would guess not.  If he's this unhinged over this, what's next?  His ego is writing checks his reality can't cash.   
 

How are you going to protect YOUR child from this messed up man?  SS?  
 

If he was truly over BM, indifference would be the mood and doing his best would be good enough in his eyes. Not competing with BM.  

It was thoughtful of you to get your SS a gift at all and unreasonable for your husband to be so negative on his son's birthday. His priorities are all wrong. I agreed with the advice to separate bank accounts and maybe not have kids with this guy right away. Those are major red flags!! I wish mine were that obvious, just saying. 

Then... file for divorce and custody in that State.

Save yourself and your own child.

Good luck. Take care of you and your baby.  Minimizing yours and the baby's exposure to this failed man, husband, and father is critical IMHO. Get the CS train started and keep it growing as the Skid ages through its own CO so when your baby is older, a huge CS check comes  your way.

You have an advantage. You are the BM, you in all liklihood will be the CP and once you are out of State visitation will in all liklihood be limited.  Use your advantages to the fullest.