[cypher]

I agree with what mangoandmimi says, the cage is small for a Syrian & the height is a problem for a large wheel, I have that cage for my dwarf hams & it's hard to get a 9" wheel in & still keep the substrate deep enough in the rest of the cage. You would have to section off a part of the cage with bendy bridges or something to keep the substrate lower where the wheel goes, flying saucers aren't good as a main wheel & that one is very small so a good sized wheel alone should make a difference to behaviour.
For now I would fill the base with substrate, you don't really need the green shelf thingy & it does take up quite a lot of room, the house looks on the small side as well, you could add in a nice big (shoe box size) cardboard box with no base for a house, add some syrian size cardboard tubes or some little wooden boxes for hideouts. You could add in a sand bath too.
If you try scatter feeding rather than using the bowl that will give him more to do.
I would think about getting him a larger cage, the barney & alexander cages from Zooplus are suitable for syrians, the alaska is ok although still maybe on the small side, the Savic Plazw is another really good cage, more expensive but worth considering.

Definitely don't use lemon or anything else on the bars.