Has he only just been neutered? In which case you may be entitled to a follow-up appointment at the vet's.
That follow-up is usual procedure following surgery -just usually a check-up to see the surgery scar is healing properly, and see all is well, or to remove sutures when needed.
However, although my own vet did that I am not 100% sure all vets operate in the same way -to include a post-surgery examination as part of the surgery cost. You can check that?
But anyway, he does need to go back to the vet and get this "lump" looked at. It probably is an area of fluid or swelling caused by inflammation, or can sometimes be an allergic reaction to the suture material. But it needs to be examined as soon as possible.
My advice is take him back to the vet who did the surgery.