It typically takes a year for a whole new hoof to grow out. Have you put him on a biotin supplement? this will help some as well.
Is he broke enough to put a small child on him in a round pen and see how he does? Maybe start with that and slowly up the weight, see how he does each time.
What does your vet/farrier say? I would get their opinion and listen to your horse. he’s going to tell you the most.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."