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This kind of behaviour isn't unusual, they can get very excited by all the new things & they get very curious & inquisitive the first few days then can get a bit reclusive as they settle down & make a home for themselves.
Unfortunately your cage isn't suitable & that may be part of the problem, hamsters are ground dwellers & need a large floor space in the cage with a large house (about the size of a shoebox) with no base sat on a deep (30cm at least preferably more) layer of substrate so they can burrow down & make a proper nest,
He's probably feeling nervous having to hide out high up & not being able to dig his way out of his house in the normal way!
They need lots of space but not too much open space so they feel safe. If you have a look in the housing section of the forum you'll find lots of discussions on & suggestions for suitable housing.
In the meantime just keep talking to him, even when he's hiding so he gets used to you being around, don't try to rush taming, it's still very early days for him & he needs time to settle.
Ideally I would suggest changing the cage as soon as you can. For now you could try making him a house out of a cardboard box, no base & door cut in one side, maybe put a bendy bridge over the door to make it dark inside & put that on the substrate in the base of the cage.
Don't clean much for at least the first month, just spot clean pee so he can get his scent on everything.