As CMB says - it can take time. It may be a slower process or it may just happen quite quickly once he has had time to settle and get over it and has learned to trust you and got used to your scent and presence.
Excellent that he recovered and you got treatment quickly.
Maybe go back to square one. Leave him alone for 2 or 3 days without doing anything in his cage (apart from putting food and water out - maybe when he's asleep).
Then don't change anything in the cage for a couple of weeks or do a clean, other than occasionally spot cleaning the pee area. Does he have a pee corner in the cage? If so you could add a litter tray to that corner after you've spot cleaned it - he'll probably use it - then you just empty the litter tray every few days and the rest of the cage should stay clean.
His cage environment can also be related to how well taming goes - if he's settled and able to have normal behaviours it helps. Clean outs can really stress them so it's best to only do partial cleans - eg do things as and when needed and not all at once.
So if you do the substrate, do the wheel a different week and any toys etc a different week again.
It isn't necessary to change all the substrate weekly - if you have plenty of substrate in the cage and use a litter tray. So 3 to 4" deep minimum. If you have 5 - 6 inches, then you can keep the clean half (usually the bottom half) and replace it and mix it in with the new, so it still smells familiar. So although you're using more substrate initially, it lasts a lot longer - it gives the hamster the ability to bury hoards and dig etc as well, which are normal behaviours.
The type of house and cage he has can help with normal behaviours too. Ideally the house should be open underneath and sat on top of the substrate, and be a good size - so they can burrow down in their nest to get cosy and bury hoards under their nest - it's also better ventilated so stays cleaner and doesn't get condensation - if he doesn't pee in his nest, then his nest can be left alone for a very long time.
So if you need to make any adjustments to the cage, I would do it now - then start from scratch wth settling in time - eg leave him alone for 2 or 3 days and then don't clean anything for a couple of weeks (except empty the litter tray).
Then start taming from scratch - don't try and handle him in the cage - get him out of the cage by lettng him walk nto a tube (or hamster ball if it fits) wth a treat at one end to tempt him in. Then lift him out in the tube (keeping your hands over the end).
If you have a hamster ball it's a good way of safely carrying them somewhere you can let them out to run around for taming. I used to put one end of the tube into the hamster ball (very near the cage after lifting them out in the tube) and they walk into the ball, then pop the lid on. If the ball fits in the cage, then put the treat in that and then lift him out in the ball and pop the lid on.
If you don't have a ball then have a high sided cardboard box or similar next to the cage and put the tube down in that so he can walk out of the tube - then carry the box to the place you can let him out to run around.
A good place to start taming is the dry bathtub - with the plug in. A couple of toys in there - eg a tube, a mug on its side for a hide, or any other out of cage toys you might have, but don't take something out of his cage.
Put the ball down in there and take the lid off - or the box on its sde and let him walk out.
It took a while of bathtub taming before our last syrian was hand tamed - a couple of sessions a week.
Your hammy will probably run around and slither up and down the side of the bath tryng to get out. While he's doing this, just gently stroke him on the back with one finger. If he jerks as if to bite then wait a while and then try again. Maybe only do this 2 or 3 times the first session.
Do it again next session. When he accepts being stroked on the back by one finger without jerking round, then move onto letting him walk over the palm of your hand with your hand flat on the base of the bath, palm up. Just put it in front of him as he walks in that direction. Then move onto lifting your hand slightly - just 1cm or so when he walks onto it but let him walk off again.
Then do that again next session. After that you can try doing that and letting him walk from one hand to the next. By then he should be tame enough for you to pick him up and hold him without "pinging" (pinging is when they wriggle free and leap 2 feet in the air at the same time!).
Even when they're tame they often don't like to sit still for long or be held or stroked for long and want to be off somewhere, but they accept being picked up, held and stroked.
A playpen area is also a good idea for taming or for regular out of cage time - you can get in it, then they run all over you as if you're an object and get used to your scent. But keep well away from the edges or they use you as an escape route and run up your arm and jump out.
There are various ways of making a home made playpen. I ended up buying one from Zooplus.
It's high enough a syrian can't get out and big enough to sit in away from the edges (just!). You can take it apart for storing somewhere.
https://www.zooplus.co.uk/shop/small...g/73299/453205
I would start with the bathtub though as it's easier to start holding them, stroking them in there as it's a more enclosed space.
Our last syrian, once tamed, would sit and go to sleep on my knee or sit for a stroke for a while (but he was quite a lazy hamster!). Our current one won't sit for more than a few seconds but likes having his tummy stroked.