If I were you, I will ask for half of the family money and keep it by yourself, Only allow him play the rest of the half money. If he can win, that is fine. If he lost money, he will not forgive himself. Don't push him to hard. He need more support when he lost money, but insist not to give him the rest of money.
Maybe he just want to make more money quickly to make family life better.
Just a opinion from a daytrader loser.