"Having a discussion with him about this and he starts to get angry." That's not a good sign. Whether your relationship is purely professional, a friendship, or a mixture of both, there has to be at least a willingness to discuss stuff, so long as you're doing it in a kind and respectful way, so that you can both agree on what's expected from the relationship.
"I feel like he’s slipping further into a 'good friend' and I’m losing control of the situation." "But it’s a weird balancing act of being a good friend and be in control of the relationship, offering a good friend financial help, and ensuring they’re ok financially." "I feel like I need to give him an ultimatum at our next meet up and move the relationship back to more business-like."
What I'm hearing is that you'd rather not do the balancing act, and you'd be more comfortable with a simpler and purely professional relationship. If that's what you want, then you can certainly tell him so in a kind and respectful way. I would avoid a big dramatic "ultimatum." Just let the guy know how important he is to you, but that you want to clarify the financial part of your relationship, and you want that part to be purely professional. Don't be afraid to be honest with the guy, even if it means the relationship may end. If he gets upset or angry, try to stay calm and respectful. Tell him that you want to stay together, but in a way that works for both of you.
I've been where you are. In almost all cases, a purely professional relationship is the easiest and best way to go. There are two guys, including one from Czechia, where we've also been friends for over 10 years. I think the only reason those relationships worked is because we were all very clear and agreed upon the boundaries. Good Luck to you!