Definitely work until he's good then let him quit. It's the best reward you can give.
I'd be really careful putting your little brother on his back if he's not quite tame yet - if he lays down when you're picking up his feet, he may lay down when he is displeased with other things, like if he gets tired of carrying your brother or if your brother shifts etc. in a way that makes the horse uncomfortable.
Safety is the number one priority. Getting injured may seem unlikely, but it's more likely than it seems. Even if it's a small risk, if it happens to you, you pay all of the consequences, the same as if it was a 100% chance. I've done some stupid things with horses and not been injured, and looking back I was lucky and wouldn't try it again. I've seen other people injured though, and it happens fast and when you least expect it. Because horses are so big they're unforgiving of mistakes, even if they don't intend to hurt us.