Yes - agree completely, you need to get his posture fixed, using therapy, and also some basic exercises that target selected areas that would have shrunk in size due to lack of use. Once his posture if fixed, he can move onto the bigger exercises such as push ups, chin ups, dips, squats - then move into medball and tempo running work then move into basic weights, then into powerweights then into plyro’s then he might be ready for some speed?
Something like that might take at his age, like 12months to progress through? Best to do it right than to rush it.