Well, I think the lump itself sounds as if it is totally normal. It's a sudden swelling of a gland along the penis towards the scrotum which happens when they are excited! Either sexually, or just generally. It happens with most male dogs.
However, the bleeding from the incision on his scrotum -keep an eye on that. It shouldn't be showing any signs of infection, oozing, or bleeding much. A tiny bit perhaps as his surgery was very recent, but I would ask the vet about that anyway. The vet does have a "duty of care" post-surgery and should be able to answer any questions you have and/or examine him as a follow-up included in the surgery charge.