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Is the room his cage is in really warm all the time day & night? It has been very cold lately & they do tend to be less active in the cold weather, even when the room is warm mine still seem to know & spend more time in bed!
Have you tried putting some substrate on top of the wheel to see if it's moved in the night? Chances are he's using it at night if it's still big enough for him to run comfortably & spins easily.
He should be putting on weight as he will still be growing, I think most of us probably give a few too many treats when taming so I wouldn't worry about that.
I would ditch the ball, most hams will get stressed in a ball & it doesn't offer them any benefit at all, if they want to run they'll use the wheel, if you don't have one in there already you could put a wheel in the playpen.
I would just persevere with the playpen, if it's big enough sit in with him & encourage him to climb on you & see if he enjoys that.
Give it time & he will get more confident.
The new cage & playpen will have interested him at first just as a novelty thing so the initial excitement does where off a bit but that doesn't mean he won't be active enough.
Are you bowl feeding or scatter feeding? Scatter feeding is one of the best ways to keep them more active.