Hi. There are a couple of responses I have seen on the forum that refer to lumps on a dog's penis ... these may be of help to you:
(1) Was the lump ... along the shaft of the penis and was there another one on the other side? If so, you are likely seeing the bulbous glands of the dog, which generally show up when the puppy is "excited" The bulbous glands are what help tie the male and female dog during copulation and are often mistaken by owners as "testicles the vet missed during neutering". Of course, these aren't testicles, but they do appear as large lumps on either side of the penis and they are very hard. Again, without seeing what you are seeing, I am just guessing, but this might be something for you to consider.
What does the lump feel like? If you feel it, can you kind of 'lift' it up underneath the skin and wobble it around? Has it got distinct, rounded edges to it? When you do this feeling around, how is he with that? Does he show any pain when you do? If it's not 'attached' to anything underneath, and wobbles around under the skin it most likely is a cyst. This can wait a little while as it's not critical, but it's still wise to get the vet to check it out. If he winces or shows pain when you feel at it, or if it has begun to look angry or inflamed, then it could be an abscess, and will need draining. If this lump feels hard, attached like it has 'roots', slightly knobbly perhaps (though not always) then it could be something that needs to come off asap. The best thing of course, is to take him to the vet for it to be looked at professionally.