Don't snitch to his mom. Yet. He's in love and kept it secret; his family screaming at him will just make him shut them down harder and go all in. You have a close enough relationship that he would confide in you, and if you snitch immediately you'll break that trust in hopes that his family can do it. Do it as a last resort.
Take him out somewhere nice and quiet. Sit him down. Tell him, as seriously as possible, how this is a bad move. The worst case scenario is that this woman takes him for everything he has, and he loses all he's built and worked for because he was enamored. Don't clown him too much on him being in his feels - he'll resist you more. Tell him it's okay that he feels very strongly about her, but you still have to pace these things. He flips houses - he understands that to make a profit on such an investment, that takes time, right? He needs to wait for a good market, clean up the house, fix it and make it presentable - something worth that much can't become liquid instantly. He needs to treat his relationship with this girl like that. An asset with high potential upside if he approaches it right and builds it right.
What if she doesn't like him as much as he likes her? Hard to believe, yes, but it's hard to see these things when you're blinded by love. What if he does something wrong and she decides to be vindictive and 'punish' him by taking his money? Would he trust you, his blood, with access to all of his accounts despite knowing you for years? If no, then he has no discernible reason to let her have access. If yes, why don't you have access then? Because the trust and relationship you've built up with him isn't based, or contingent, on money. He can be popping 200k in his savings or be broke, but either way you'll still be his cousin and he can still trust you. And he should seek the same thing in his romantic relationship. If she suggests a joint account as a basis of trust, can she really be trusted? Doesn't that move go against all of his other successful relationships?
Tell him that you look up to and are proud of his achievements considering his circumstances. You saw him lose a father and take what he got instead and build it into something. And it would be a massive shame for him to lose it all on such a gaff. It's not a guarantee that she'll do that to him, but it's a possibility. A possibility with no recompense.
If he becomes more receptive, tell him to secretly make new personal accounts and move his money over. He can leave a small portion of it in the joint account as a show of trust if he likes - as something they can start off on and build on.
Do not tell her he's doing this; again, worst case scenario is that she takes everything in the small chance that she really is only with him for the money. Once he does that, if she asks, he can tell her that he doesn't want their bed of trust to be built on his income. It should be built on character, on experiences. If she's truly into him and wants more, she should respect that. If she doesn't respect it, and pushes for access despite not having done anything to earn it (or contribute), then perhaps she's not the woman he thinks she is?
I must stress to you that while the coli is right and your cousin is absolutely wylin, you can't take that approach with him. Whenever someone is shytting on you for acting up, don't you naturally wanna wyle out again just to spite them? If it dawns on him that perhaps he was hasty, cussing him out won't make him want to fix it faster. Instead, use the trust he's placed in you to guide him out of the simpin' place.
If none of that works and he's still adamant on keeping this girl on all of the accounts, then alert his mom and the rest of his family.
Good luck.
I hope you trolling. If not you should pull him aside and explain that he needs to take her off because she is not someone that deserves that level of commitment at the moment. 4 months is not long enough to know someone's true intentions and the fact that she was bold enough to ask him to do this is a MASSIVE red flag.
