Sounds like maybe he doesn't have the strength required to fly yet from not using his wings in such a long time. Maybe you could perch him on your hand and run with him to see if he tries to open up his wings and flap them, it won't get him flying right away but it might be enough exercise to re condition them and get their strength back.
I'd be interested in what others have to say here too since we just adopted an

last week that's been clipped to the point she can't even do a soft decent let alone try to lift off so I'm guessing it'll take 6 months to a year before she gets her flight feathers back and will most likely end up in the same boat yours is in.