Worried about one of my hamsters. He pretty much stays in his tower and has never run on the exercise wheel.?

I have 2 Syrian hamsters, both 4 months old. The female seems to be fine. But my male called Astro, seems to be having a few problems. I am seeking advise and to find out if he is behaving normally or not. He is very nice and lets me pick him up or eats out of my hand without any problems. He is kinda lazy though and I have not once seen him get on the exercise wheel. He is in a Here and There 2 cage with a Crittertrail Tower on top of it. He pretty much always hangs out in the tower and never wonders down. By comparison to the female he hardly ever wonders about. In fact I have worried that he wasnt drinking and went up and took the bottle to the tower to make sure he was taking drinks. I have an exercise ball and he doesnt seem to enjoy it much. Yet he is just so loving when I hold him and looks at me and is so not afraid of me and will sit on my hand without wiggling about, like she does. I feel duty bound to ask for help, especially since he is such a nice hamster.