My Husband Has Painful Lumps.. One At The Back Of

My husband has painful lumps.. One at the back of his neck.. And three are on his buttocks .. He said the lumps seems to be under his skin so it cant be easily seen. My husband is a chain smoker and is on his early 30's. He is slightly overweight as well. What can be the cause of his lumps? It started with just one at the back of his neck.. Sometimes pain will stop but the lump will not disappear.. Then will suddenly be there again after a few weeks or months.. And now it had progressed wherein he has 3 lumps under the skin of his buttocks. What's happening? Please help