I wish I had better news, but in all honesty from both personal experience and what I read here, it gets worse, not better. It sounds like the BM in your situation is high conflict, and one of the things making it worse is your BF's family. If they are the kind to gossip and our contact with her, that will continue to add fuel to the fire. Also, the fact that you don't have a CO in place and the boundaries that that creates means your BF will continue to feel like he doesn't have a leg to stand on, and he'll continue to bend to her wishes and have to deal with her drama in order to have access to his son. but you really and truly have to prioritize that.

not that lives can't change all the time, but consider waiting to get married until things like a custody order or in place, and you can get a sense of what your life might look like going forward.write out a best case scenario in five years, worst case scenario in five years, and no it will probably be somewhere in between, but that can help you see and plan for what those realities might look like, and help you decide what you need to do to protect yourself financially and emotionally.

Is your boyfriend paying child support for his son? Has a paternity test ever been done or an affidavit of paternity signed?  It is possible to establish paternity, file for visitation and set up child support without a lawyer.  Your BF can call the clerk of the court to find out what paperwork needs to be filed. Without a court order, the BM will always be using the child as a pawn and there will never be any certainty.  Has your boyfriend at least been saving up for a lawyer so he can get this straightened out?  The longer he waits, the harder it becomes and the more "status quo" is not in his favor.  His child deserves to spend time with and be supported by both parents, and with a HCBM, he's going to need a court order to effect that.

In your shoes, I'd lay low, avoid contact with the BM, let the boyfriend do the parenting, and do not marry him until he has a court order.  Otherwise your life will be subect to the whims of BM.

Yes, file without an attorney, it can be done. In my state the forms are online. 
 

Beware, this is the type of woman to dump the kid on you two when she can't use him as a pawn anymore. Which right now probably sounds good to you, but adds a whole layer of stress to everything. 

I think a condition of you getting married needs to be that your BF figure out how to legally handle BM.

He doesn't need an attorney, but he needs to do his research. If he wants an attorney, then he needs a second or third job to save up money to pay for one. 

If you can't keep dealing with this, then you need to put the pressure on the person who has the responsibility to handle his own crap, and that's your BF. As much as you look at yourselves as a team, this is one of those arenas where he is playing solo. Think of it like wrestling: you're on a team together so you can support one another and cheer from the sidelines and offer support when they don't win their match. BUT, a single bout is fought between two people, and you can't be the one that goes in and fights that fight for him. And you have your own bouts that he can't get into the ring and fight with you, like anything to do with your own family or friends.

Sit your BF down and tell him this needs to get sorted. You'll help him with filling out paperwork, you'll attend meetings with attorneys, you'll take on extra chores if he needs to take on another job, etc. But HE has to wrangle this in with a court order. HE has to get child support settled NOW. And for your own sanity, you need to wait at least a year after he does this to see what kind of crazy pops up from BM after he establishes custody.

Him not doing it now opens him up to liability in the future. What if this child isn't his? He waits too much longer and he may end up paying CS for a kid that's not his, without ever getting to see the kid, or the kid knowing BF isn't the dad and throwing it in BF's face every time he sees him. That may not seem like a big deal now, but wait until you have your own kids (if you decide to have kids, that is) and you watch money float out the house monthly to be spent on a child that isn't your BF's while your kids miss out.

Or, if BF really is the father, BM drags this out and wants back CS from the time the child was born. Now your BF is paying double each month to pay his actual CS plus arrears, AND any money he gave her prior to the CS order being in place will just be seen as a "gift" and not count.

The custody piece is its own separate evil, but the longer your BF waits to file, the less likely he is to get anything more than every other weekend. If he has texts or emails from BM stating she agrees to 50/50 custody (or every other week), great! If he has texts showing him asking to see his son every other week without fail, great! Compile all that evidence. The longer he waits, the harder it's going to be to convince a judge that he actually wants to see his kid and doesn't just want a reduction in CS.

This NEEDS to be settled before a wedding, house, new car, or any other big purchase is paid for. If that saddens you to hear, then you need to re-evaluate your relationship. Reality is, step life is expensive. CS is outrageous much of the time, custody agreements for fathers are generally awful, and these high-conflict BMs will do everything they can to make life a living hell. Your BF's job, if he wants to have a relationship, is to keep all that as minimized as possible. That comes with CS and custody orders at the bare minimum. It comes at limiting communication to email. It comes with filing contempt in the courts every time BM doesn't follow the CO. It comes with celebrating holidays and birthdays at weird times of year. Step life dealing with a high conflict ex is MUCH different than the Brady Bunch-style step life portrayed on social media. Those rules of engagement don't apply.

Finally, if you're going to stay in this, decide now if you'll disengage from your SK or if you'll mentally give BM the finger and do what you think is best. BM is going to be upset no matter what you do, so you either have to decide to do nothing or not let it bother you. BM in my life used to always make a big public display if I ever tried to interact with my SSs in a way that showed I knew something about them. She always "knew more" than I did. Well, now my youngest SS lives with me full-time and my oldest stays with us when he is home from college. She knows very little about them in spite of what she claims, and while I didn't like her calling me out for seeming too "motherly", I'm glad I did it. There are other folks who preferred to disengage, and that was the right call for them. You will have to decide the same.

You really only have one option and that is to file for a court order. You need to have boundaries in place and she is not allowed to control yours and BF's home- any more that you can control hers. 

This child is young, You have a very long road ahead. Get the legal stuff worked out, there is no need to delay it. You have proof that he has been with you both 50% of the time. Just make it official so she can't play these games. 

You don't need an attorney to go to family court. Also if he doesn't have the money he may qualify for a court appointed attorney.