I met my DH when SS9 was 10 months old. I did not meet SS until he was 2. I was always known as "Dad's girlfriend." I didn't become "stepmom" until shortly before we were married, when SS was 6. DH just explained to SS that when Dad married CastleJJ, she became stepmom and SS became SS. We just chalked it up to marriage saying that marriage made us family and gave us new titles. It was a pretty normal conversation and there weren't a lot of questions. 

But you aren't SM, so not even a mothers day card was appropriate IMO.  You can be dad girlfriend, which is what you are-though at age 5 they probably don't know much the difference between dads friend and dads girlfriend.  Don't be in a rush to solidify any kind of title, it'll bite you in the as* and make things harder.  Don't be too eager to become SM.

It is great that BM seems welcoming to you and acknowledged you! Most SM's don't get that!

As for being Daddy's friend vs. Girlfriend- don't worry about making it an official thing. Are you living together or plan to soon? If so, I am sure the kid has figured it out. 

Once you get married or are at least engaged, then start the SM stuff. Prior to that, there really isn't any need. 

And just for your own sanity- remember, this kid has 2 legal parents. You are not a parent or legal decision maker even when you get married. You get no say in anything that the father doesn't agree to. That doesn't mean you don't matter, because you do. It doesn't mean that your BF shouldn't consult and talk to you about schedules and changes any everything that will involve you and your home- because he should. But too many new SM's get excited and think they have a say and just get hurt in the process... 

Even with my special needs SS21 who lives with us. His mom has never had more than supervised  visitation since he was 1. He lists me as mom, I go by mom outside of the home (at home he calls me by my name)- however the moment someone finds out I am only SM, "they need to make a note", "SS, are you sure you want her here for this?" , ect. Thankfully, he looks at them  like they are crazy and insists I be there, but my point is, the world will try and make you something apart from them. You are just a step parent. 

The flavor I get regarding your situation is that it should wait until after the wedding. 

In our case, I met my bride when SS-28 was 15mos old.  I was "Dad(dy)" not long after.  I was the first person he ever called Dad(dy) and I am the only REAL dad he has ever had though he always has known and had a relationship with his Sperm Idiot.  Long story.

SS chose to call me daddy. It was never a mandate.

The term Step Dad never entered our family language lexicon until SpermGrandHag started F-ing with SS's head with the REAL dad, Step Dad crap.  

When you become Step Mom (in title) is up to you and no one else IMHO.  The breeders of the Skid are irrelvent in this IMHO.  Certainly your mate gets an opinion and you should discuss it with him.  BM... she gets no say in shit for anything in your home/relationships including your relationship with YOUR Skid.  He is YOUR Skid, he is not BM's Skid, nor is he your BF's Skid.

IMHO a SParent has to defend their role and their place from anyone who would overstep to challenge their Iplace. Regardless of why they are.

Go slow. Little kids get hurt easy. Had a coworker who always had her little girl call her boyfriend daddy. After about 3/4 years they split up and it broke the kids heart. He wasn't really daddy so there was no reason for him to ever come back around. Little girl was 5.

most people don't start with the step parent thing until they are engaged/married or living together long term. You usually don't have to have a formal conversation with a kid that young. At 5 all he really needs to know is you are special and loved by his dad. If you have a more permanent arrangement with his father than just dating, it's probably enough to let him know you are another adult who loves him and he can count on. Let the rest develop naturally and don't worry about the title stepmom, it doesn't come with many perks anyway lol

The flavor I get regarding your situation is that it should wait until after the wedding. 

In our case, I met my bride when SS-28 was 15mos old.  I was "Dad(dy)" not long after.  I was the first person he ever called Dad(dy) and I am the only REAL dad he has ever had though he always has known and had a relationship with his Sperm Idiot.  Long story.

SS chose to call me daddy. It was never a mandate.

The term Step Dad never entered our family language lexicon until SpermGrandHag started F-ing with SS's head with the REAL dad, Step Dad crap.  

When you become Step Mom (in title) is up to you and no one else IMHO.  The breeders of the Skid are irrelvent in this IMHO.  Certainly your mate gets an opinion and you should discuss it with him.  BM... she gets no say in shit for anything in your home/relationships including your relationship with YOUR Skid.  He is YOUR Skid, he is not BM's Skid, nor is he your BF's Skid.

IMHO a SParent has to defend their role and their place from anyone who would overstep to challenge their Iplace. Regardless of why they are.

To be honest I would be kind of annoyed if I was still Dad's friend after 3 years and not his girlfriend unless you two have not being making long term plans at all until now.

As for the step mom thing - I think that when you and your SO are sure that this relationship is for the long term and are making plans as such then it could be appropriate. Marriage is an obvious stage, but I think that even moving in together is an appropriate stage if the kid is seeing you and Dad live together. 

Personally I think I only really introduced the term step-mom to my SD when she asked something along the lines of ...will you sort of be my mom?...and I replied with something like - you already have a mom, but some people would call me your step mom because I am married to your Dad. (this was a good few years ago now so I can't remember the exact conversation).