That would be a great thing if you could help him feel better about himself, and in a way overcome some of his disability even if only in a psycological way. But, I would advise you to make him clear it with his doctor first. It really depends what is going on with his condition weather it's advisable or not.
CP is an umbrella term, PT is common to help with the condition but he might be limited in types of exercise or amount of weight he should be lifting (ie, it might not be good for him to lift like you do). For example, onset of arthritis and osteoporosis can occur much sooner in adults with cerebral palsy.
So just have him talk to his doc first.