IF it is a hernia, I wouldn't rely on his pain threshold. My hamster was fine, until one day I picked him up, felt a button on his tummy and the 'lump' had gone rock hard and was bright blue underneath the skin. The vet said it was a undropped testicle when I queried it (and he's usually reasonably thorough, especially as - like yours - he only had one visable)
I suggest you (gently) keep an eye on the feel of it, if you notice any stiffening of it, or colourchange, get help immediately.
I hope it is just an undropped skadoo, that'll make it all much less complicated!
Well good luck!
(And I don't mean to scare you - it's best to worry about it and double and triple check, than not check at all though, right?)