But ultimately, it is your decision. If you think he is well and active now then a second opinion wouldn't do any harm. I do remember a hamster on here (also called Nelson I think!) who had a lump removed/surgery at about two and lived out his life for another few months and passed peacefully. I don't know if it was definitely cancer or not.
Hamsters do tend to recover well from surgery, if they survive the anaesthetic although the anaesthetic can affect them as well. Our 2 year old syrian had a whiff of anaesthetic for a tooth trim and wasn't so good afterwards.
I think only you know how he is in himself. In removing the lump it may cause cancer cells to spread elsewhere which is why it often comes back.
It may be a slow growing cancer and not bother him for some time. Very difficult decisions. Although you know, and are distressed by the diagnosis, it can help to remember that the hamster doesn't know
