It's hard to tell if it's cancer or not. Have they grown really fast? Or has it been slow? Usually cancer seems to grow faster than benign tumors. Both my hamsters who had cancer had one massive lump that first grew very fast, then slowed down, and then grew really fast again. Also, if you can touch it and it feels rough underneath the skin (as in not smooth) that's usually a sign of cancer.
The fact that he doesn't seem unwell is a good sign
Both my hams were healthy for a very long time after they got cancer, so even if it is cancer he might have a lot more time left