I think my cat is dying...?

When I was little my family adopted a stray cat. No one really knows how old he is but we've estimated about 13yrs. He's a really sweet, lovey dovey cat but lately he hasn't been wanting as much attention as he usually does. He is very bony and has seemed to lost much weight, he seems disoriented and often can't tell where I put his food when I set it down for him, he also will walk like he doesn't know what's going on. He rarely uses the litterbox anymore and his fur is all clumpy like he's dehydrated. He'll sleep in random places, mostly near his water bowl, and earlier I stepped on his tail but he didn't whine or anything. He barely makes any sounds anymore, not even when you accidentally hurt him like I did. He also wanders around ther house aimlessly and will circle the rug in my room for a while before deciding whether to come in bed with me or leave. It takes him a long time to walk up our basement stairs and he seems very weak. I feel with these symptoms he's getting ready to die. If he is dying, should I take him to be euthanized or let him die naturally? I am scared he is hiding his pain and I know cats have a high pain tolerence but I don't want him to suffer. Please help.