Thanks for the replies. I thought it might be a tumour as well. It's hard to get a close look at it because he's a lively little thing!

I just saw him standing up against the edge of the tank with the daylight shining on him so I managed to get a closer look. It's still growing, and it definitely looks like 2 lumps merged into one now. I can see his heart beating through the lump that's highest up on his chest. It looks as though his heart is in the centre of he lump, protruding from his body. The second lump (attached to the 1st) looks like another organ protruding from his body, but I might be wrong.

He still doesn't seem to be suffering at all, but the thing is getting bigger so he might start suffering soon