Please do get him seen by the vet, but a soft, movable lump
does not necessarily suggest a cancer to me. I have a very lumpy dog
, she has some lipomas (fatty tissue) which are soft and movable, and gets cysts (fluid-filled smallish ones) now and again, which come and go. EVERY single new lump gets checked out professionally, but I am becoming a lump-connoisseur (is that how you spell it?) with my dog, and have got used to how all these different lumps feel.