My hamster is losing weight, is he OK?

My best guess is that he lost weight due to being ill and that he's probably not very fond of any of you right now because of you having to administer the medication. I would keep an eye on him and make sure to give him time and the opportunity to bond with you or to at least earn back your trust. Hamsters aren't always the most friendly critters in the first place. If your hamster starts showing other signs of illnesses, bring him back to the good vet. Otherwise just do little things like read next to your hamsters cage and offer his some treats now and then so he can see that you're not just connected to horrible medication.