I watch a lot of odd TV shows, and on an episode of Dr. Pol, he treated a horse who had what looked like a tumor under her tail as well. It was a soft tissue tumor(not cancerous). But what he did was put a casteration band around it and let it fall off. Since you can’t really cut them off without causing a lot of bleeding and discomfort.
Maybe talk to your vet about this option? There has to be something you can do for him. I don’t see any harm in giving him mineral oil.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."