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The P@H cages aren't really suitable for a syrian, even a small one so you might do better to just get another cage, the Barney cage or the Alaska, both from Zooplusare quite a reasonable price & a good size so you'd have no trouble putting your trixie wheel in with lots of enrichment too.
I did see some photos but they seem to have disappeared now (I can see them on my ipad but not on my computer which is a bit weird!) & it doesn't look as though he has much to do in his cage & he does need a lot more substrate, completely fill the base, so he can dig & burrow, you could just use things like bendy bridges to make some hides & things to climb on for him, otherwise have a look around the forum at other peoples cages for more ideas.
He could do with a bigger house too really, the house needs to be on the substrate with no base to it so he can dig down & make a nest rather than it being on a shelf like that.
He may still put on a growing spurt but he probably needs a good size wheel & more enrichment in his cage to keep him active & help build him up along with a good diet including sufficient protein for a young ham, what are you feeding him now?