G.B.
Seems like there are community organizations who purchase things like this for people who need help. I know they bought a tricycle for a man I know who was hit by a drunk driver and nearly killed. He rides the tricycle to work sometimes and around the neighborhood to strengthen his legs and help his balance. It also helps him be able to walk. My cousin has a horse ranch and they work with kids who can't walk, the motion of the horses starts their brain going and it helps them to be able to eventually walk.
I think if you are taking him to PT hey might be able to have resources we can't even magine.
I googled walkers for kids with cerebral palsy and got this website. I think it looks very promising.