What mix do you have him on now?
I would suggest trying a different mix, sometimes hamsters can get a bit bored of the same food so it's also nice to provide them with veggies/fruits or even a piece of cooked chicken, not too much of anything fresh of course as it will just go off if he doesn't eat seeing what else he will try just as the occasional treat!
If his normal mix is completely pellet based and you don't want to change it up completely, I would still try to buy another mix and add some of it in to give him a variety
