Bless his little red bum!
Your doing great arfey. One thing I would follow now is this. Because of his past and the fact that he`s jumpy or may be feeling a little strange, keep cage cleaning to a minimum and only spot clean. Don`t throw any of his shavings or bedding away, even if you feel it needs to be. Syrians being solitary hamsters rely on their instructs to build a home, a secure territory and that to him is his cage, his boundary. He will feel safe in there.
Keep everything basic (as you are doing) and let him be the king of his castle. He may well stash food too so let him. That is him hiding his stores for future use. If a hamster ever pees on it, that usually means they know it`s been disturbed. I always leave my dwarf hamsters hoards alone now! x