Maybe do some walking and slow trotting with him, walking next to him not riding, I knew a horse who wasn't ridden in 7months because of a tendon injury and she was perfectly fine, but that was just that horse, idk how your horse would react ☺
I suggest gentle ground work
Listen to your vet!!
He is educated and is the "expert" here. By riding your horse before the vet says it's okay, you can cause some serious pain and problems. If the splint is not healed when you start riding (trot/gallop), there's a big chance that you could make it all worse and the horse would become hurt again, maybe even with a worse damage.
You could do some mental training with your horse, "forcing" it to use its energy to think. You could also walk him, and if you stable has a walker maybe put him in it. Just make sure he is as little bored as possible.
My horse was off for 6 months with a tendon injury and when I got back on she was beautifully behaved and not wild, I'd personally listen to the vet to prevent further injury but as others have said maybe you could walk him or give him some time out of the stable
You should spend time with him and reforce his confident with you
Maybe walk him or/ and a little bit of trotting if your vet says that would be ok and he doesn't seem to be in pain or reluctant at ALL by hand
Well if the vet says let him rest let him rest because you don't want t cause pain?
As the other comments say, try exercise and maybe some light lunging
Try and do exercises with him.
I doubt he will become "wild" maybe just walk him inhand and do some parelli games with him, that way you can use this time to bond with him and work on issues from the ground☺️