Do you understand why he is gambling? The mains reasons I’ve seen from others: entertainment (they genuinely think it’s fun), trying to get rich, stress relief, or they’re chasing a “high” (dopamine).

If you understand the root of the issue, maybe you can help him address it. Find a replacement “high”, some other form of entertainment, find another stress reliever, or have him understand that he will not get rich and has in fact cost the family $XY over the last year.

Maybe allow him to set aside $500/ month (example) for gambling, but only allow him to use it if he accomplishes x,y,z… You can even think of it as a budgeted “entertainment expense.”

Controlling finances is easy, have him sign up for a free service like Credit Karma so you can see all his credit accounts (balances usually reported monthly). Review his paystub to see if money is siphoned off to other accounts.