What's that smell and where is it?

I have a serious problem going on in my oldest son's room. It STINKS! I'm not talking the normal little boy stink that goes along with having a boy. This smell comes and goes and we never know what we will face when we walk into his room. We have all been trying to locate the source of this smell for months. At first I blamed it on a dirty fish bowl since it smells a lot like fish food. But after cleaning the tank the smell would still come back. But we got rid of the fish, just in case. Mainly because I was so tired of having to clean the bowls out but that's another story. So then I thought maybe fish food was spilled somewhere and just needed to be cleaned up. I have taken that room apart, I have vacuumed and used gallons of ammonia to get rid of that smell. My husband has looked up in the attic in case something died up there but found nothing. So what the heck is this smell and where is it coming from. I have noticed that it only starts to smell when the light is on in his room. We can run the ceiling fan (to air it out) without having the smell return. I truly don't know what to do about this smell anymore. I feel so bad for my son although he doesn't really complain about it. At this point in time I would gladly trade this yucky fish food smell for something more desirable, you know like sweaty gym shoes.