I have seen this on several of our cattle. It always seems to be an infection. Most of which are caused by a tiny sliver of wood from feed bunks. We usually stick a needle in it and let it drain. Really disgusting stuff comes out. Then we give the animal antibiotics. If untreated, the lump can kill the animal. I suggest you sneak up on the deer in dark, rope it, and fix him up good. But if you're not as big and strong as a tank, this might not be a good idea. It looks like the deer has lostsome weight. Maybe call a conservation officer to put him down and out of his misery. On the other hand, deer are amazingly resilent and he may come out of this just fine.