Would it work to explain to him why casinos are bad?
"Games that are like this (point to app), like casinos, have "random" endings. "Random" means that sometimes you win, and sometimes you lose, and if you were to play many many times, and count the number of times you win and the number of times you lose, they would stay at similar levels. For example, you win two times for every one time you lose. But, here's the trick: these games are made so that they trick people, even very smart people, into thinking they win more often than they lose, but they actually don't! These games take over your brain and make you think things that are not true. And if you let them do that, you will lose more often than you win, so overall you will lose and that is bad. So it is best to stay away."
When he is older, yes I'll need to pull back a bit :)