[Petite]

I can't help wondering if he has been abused in some way by the previous family (although that may be completely unjustified). What did the vet say about his behaviour?

One would normally expect a hamster to be well settled by this time as you have had him since February. Have you tried feeding him through the bars of the cage? If he learns that you are a source of treats, he may stop screaming at you. Of course, it is possible you have got one of those hamsters that just doesn't want human contact, and it's also possible that that is why the last family gave him away. Has he behaved like this since you first had him?

If you had confidence in the last vet, there may be little point in taking him to another, unless he is showing signs of distress (sitting hunched over in his cage, ears pressed back, or running in circles). Does he behave normally when he thinks you are not around? And have you tried taking him out of the cage for taming sessions, so that your hand is not in his territory?

Sometimes, we just have to love our hams for what they are, although it can be difficult to love one who doesn't want to connect with you, but please don't feel sad or stupid - it sounds as though you are doing everything you can to give him a happy home.