I will also say that while it is admirable that so many people are truly concerned that the man may be in need of aid due to illness or injury, the signs don't really point to that. The OP's description of the house, the cigarette butts ground into the floor, the clothes all over, the trash, it all points to a slovenly person. If someone was simply hurt, the house would be dusty, there may be bugs or rodents, the mail and papers would be piling up outside, and there may be dishes from the last meal in the sink. And, to be brutally frank, if something REALLY horrible had happened, there would be a smell. I REALLY think he's just a pig. But certainly call the police if your conscience won't let you rest.
My husband had a coworker a few years ago who fell off the bed one night and slipped between the nightstand and the bed. He had been drinking, and didn't wake up, and the pressure of the nightstand closed off his breathing and he died. Everyone at work wondered where he was, even his close friends, but they figured he just blew off work. After three days, one of them went to the door, noticed a smell, and called the police. The neighbors were not the ones who should have been checking on him, his friends and coworkers, the people who knew him were.
OK-I am sure I've totally alienated someone, and I am sorry for it. This is a topic that has managed to ALWAYS affect me in some way, and I feel strongly about it. I hope that the man is fine, that he calls the OP, they laugh about it, and everyone gets together for a picnic.