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I would just stop trying, I've never felt the need to hold my hamsters lying on their backs, if he doesn't like it then just don't do it!
As long as you can handle him normally then why worry, if you persist with something he doesn't like then you may find he will get nervous about handling generally & you're taming will go backwards.
Let him climb onto your hands & sit on your hands if he will, that's all you need do.
I would never blow on a hamster, if they bite then I just take a break for a while & then go back to what I was doing, if they bite persistently doing something like this then you know it's not going to be something they like.
There are times when you may need to hold a dwarf ham in that position, for example to check a males scent gland or to check their teeth when doing health checks but if you just get your ham comfortable with normal handling then it's not usually a problem at all to briefly hold them in that position.