Here's a different tact - rather than reminding him to not slouch, try helping him build muscles so he automatically won't slouch.

I have a history of slouching and see it terribly in my father. So I was pleasantly surprised when I started doing exercises that strengthened my shoulders and upper back, to see my tendency to slouch disappear almost like magic.

Often we slouch because it's how we relax when our muscles are weak. Building the muscles that pull his shoulders back will help him stop slouching with less effort.

On the other hand, as a parent who believes in telling kids like it is, I don't see any harm in talking to him about it, reminding him and working with him on it. You can make it a game to see if he slouches after 10 minutes - create a chart, etc.

Most importantly, you can explain the why's - how it will help him feel and look better as he gets older.

I hope that helps.