Hello. I'm sorry to hear your little boy has been diagnosed with Cushings. Our last Syrian was as well, at the age of about 2 years and 7 months. However I was not sure that it was actually Cushings as it is normal for hamsters to have some hair loss when they're older. The main thing is that he is comfortable. He is very old for a hamster. The equivalent of about 100 human years old
Do you know what the medication given is called? How you are giving it sounds just fine. Ours didn't like taking anything from a Syringe either and it's important that they aren't bothered too much as well as giving the medication - it's a balance.
I wouldn't worry too much about the disease at his age as he is clearly starting with it in very old age, if it is Cushings. So he is unlikely to get some of the symptoms younger hamsters get, which take a long time to develop.
It does sound like he is in his twilight time. Is he still active and seem well in himself? Coco's resource is very good.