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Some Russian hams can be a bit bitey, they often grow out of it & get better with time. Hamsters aren't aggressive, he may confused by some things, once he's become used to your hand offering a treat then stopping the treats isn't a good idea as he's learnt to associate your hand with food.
The change in his diet won't have been good for him either, never remove anything from your hams mix, if you give a good mix picky eating isn't usually a problem, seeds are a natural & favourite food for hams so he will want those, if the food has pellets or biscuit bits in he may not like those so best to go for a good mix without those.
I would avoid putting your hands in the cage any more than you have to while he's in there, if you need to do some spot cleaning put him in a carrier with some substrate or if you can keep an eye on him do it while he's in the play pen, they can get territorial over the cage & really don't like it being messed with!
He sounds as though he's really quite tame already so keep interacting with him out side the cage, they do love to climb all over you so enjoy that & with time he should begin to nibble less.