In my opinion, never force him to. It's very unnatural for most birds to lay on their backs so it's best taught through trust.
What I did with Peanut when I wanted to teach her:
If he likes cuddling, wait until he's calm and you are holding him while he's cuddly. Keep one hand under his feet and place the other on his back (if he doesn't like a hand on his back either teaching him that's okay and a good thing would be step one, meaning he needs to trust you more.)
Gently and slowly roll him towards his back but don't grab or force him. Also rolling him to a side may be less threatening to him if he doesn't like being put on his back at first.
If he wants to leave, let him. Pay attention to how comfortable he is with what is going on and try to stop before he's uncomfortable and wants to leave, then stop and reward him when he does good!
Even for a bird that doesn't like to be on their back eventually he should let you roll him further and further until he'll eventually lay in your hand with his feet in the air while you praise or cuddle him.
Some birds are more prone to laying on their backs for cuddles and play than others. Peanut does not really like to be on her back at any time but will remain relaxed and keep cuddling if I gently flip her this way.