I'm confused. Your last post was in April and you were married then, so I assume you are talking about a prior marriage?

First off, how do you know he's more bonded? If it's because of social media, take it with a grain of salt. 
 

How old is your child with him? People do change over time and it's possible he's more ready, or in a better financial position to be a more involved parent now. 

I'm the new wife in this situation here and I can tell my DH is definitely more involved with our daughter's upbringing than his eldest. His relationship with BM was different than his with mine for sure but I do think he is in a different spot now than 2 decades ago.   Back then he and BM agreed that he should be the primary breadwinner while she stayed at home. That recipe led to a lot of resentment on both sides because SD was a stage 5 clinger with only BM as a young child so DH always felt like the outsider when he arrived home exhausted. From BM's point of view she felt like a single parent because her husband was never around. Sometimes that arrangement works beautifully for some couples. With those two though it led to the shitshow of the last 20 years.

Nowadays we're more laid back so there's a lot more family time. We don't earn as much but we don't spend as much as he and BM used to. It's just different times, different things register as important.

Don't take it personally, I think people who have kids later (like in their 30s) end up being better parents for a lot of reasons. Though parents getting along with one another does surely help.

They tend to have more money, are more stable, more mature, have finally picked a good partner, kids are more likely to be tried for than to just happen, and have had enough exposure to kids to learn from others' mistakes, and in the case of remarried bioparents, their own. 

Everyone I know who has had a kid in their 30s is a much better parent to them than people who have had kids in their early 20s. 

My DH definitely learned from his mistakes with SD and BM. This time he's picked a more supportive partner (me!), We tried for our kids rather than with SD when BM decided on her own without talking to DH. He's more mature and patient, and the sum of all this choices is making him a better father to ours than he was to SD.

How long were you and your ex together after your son was born? How old was your son when you two split? How often did your ex have him after the split? I think these are all huge factors that impact bonding. 

My DH had SS9 with BM when when were both 19. Now our BM is high conflict and has done everything in her power to prevent visitation and bonding, which makes this scenario different. DH did not have overnights until SS was 5, but had visits a few times throughout the week. We now operate on a long distance schedule of 6 weeks per year due to BM moving out of state. DH and I are now expecting our own child, and DH admits that it is way different. It can be hard to be bonded in the same way when you see one child every other weekend or every few months, versus a child you see every single day. My husband admits that he feels more like SS9's "fun uncle" than he does his Dad due to the lack of visitation and involvement. Also, depending how young you were, your ex may not have been as mature as he is now. You say your child is a teenager, so it has been plenty of time for your ex to grow up. 

My exH is a much better father to his kids with his wife now than he was to ours. He has openly said that it took him losing his first family to understand... so maybe that is what is happening here too?

I think my first husband was a better dad to our second child than our first. It was a function of a lot of things -- school, work, available time, his experiences growing up, etc. What it wasn't was a reflection of love. There are lots of moving pieces that go into parenting. Things don't stay the same from one child to the next, even with the same parents. 

The steady constant in all of this is that what is portrayed on Facebook only reflects what the poster wants the world to see. And many people want the world to see their best --even if it is fake. (See halo1998's post about BM and homecoming)

For sure the quality of the bond between the parents influences the quality of the bond between the parent and the child.  Particularly on the NCP side of blended family relationships.

The constant manipulation and attacks by the SpermClan against his mom severely damaged my son's relationship with his SpermClan.  So much so that he has nearly zero to do with them as a successful adult.  
 

The three of us are extremely close.  

I have observed the exact same thing with my DH and believe the cirumstances/environment you bring a child into play have an enormous impact on ability to bond. I also think personality plays a role ie we bond easier with kids that are likeable or share similar traits to us etc. 

My DH had a dysfunctional and unhappy relationnship right from the very start with the mother of his first two kids. He was very young when they got together and BM was 11 years older - creating a huge power imbalance. He also had to move countries to be involved in his kids lives as BM moved back to her home country when pregnant (something he resented enormously at the time as he didn't want to leave the UK). Bottom line, he was unbelievably depressed and by the time he matured enough (via age and therapy) to realise how bad the situation was his second child was on the way. He ended the relationship shortly after his second child was born.
 

As a result of all of the above, it took DH a long time to bond with his kids whereas he has been so unbelievably smitten with our DS3 since he was born. There is no question that DH has the strongest feelings for DS3, however he does very much love the other two and is a very active father in their lives (50/50 custody) but it's not the same. I would also say there is a difference in how he feels about the first two. SD16 is difficult and unlikable and DH is very dissapointed with the person she has become. SS15 is a gorgeous kid and we both love him very much. SS15 is also a lot like DH, whereas SD is a clone of BM (who we both hate) so that doesn't help. 

I know DH loves all three of his kids but there is no question that he is most bonded with DS3 and then SS15, with SD16 last. People will criticse but bonding isn't something you choose, it depends on a range of circumstances (ie environment, mental state, likeability of the kid, things in common etc). What I do know is that the skids don't question their father's love for them, he's very loving and affectionate with them. 

I think my key concern would be does you son also sense the same? Does he feel rejected by his father?