Sounds like a potential red flag for gambling addiction from here. Could be something else but it’s worth checking out. Gambling addiction is easy to hide as it doesn’t affect your physical state like drugs/alcohol, but still changes personality and leaves the addict unable to consider anyone except themself.
(His drinking doesn’t sound too healthy either, tbf…)
Have a look at finances if at all possible. If he’s told you he’s lost two grand, there’s a big chance he’s lost a lot more. Check for debts (credit karma or one of the other apps that shows all bank accounts, loans etc in someone’s name).
It sounds like you absolutely know that something isn’t right. Follow your instincts, check up on anything that smells wrong and don’t be fobbed off.
Even if you don’t find out what’s causing this change, you don’t have to put up with it. If his behaviour doesn’t improve, set some really clear red lines on what you’re prepared to tolerate, and what you’ll do if he breaks those boundaries. Make them realistic, so you can go through with them. It’s not a nice thing to have to do, but you deserve way better.
Finally, if you haven’t already, talk to friends, family, and make sure you’ve got support in place.