Judge on

OKAY people lol. I've got a six-year-old guinea pig that just will not die. I know that sounds harsh he was a pet for my son a very long time ago and we have taken good care of him but there are days he won't eat anything anymore and he starting to make all kinds of really loud noise all hours of the night for no reason and is really mean now. I'm about to have a baby and my son doesn't even want the poor thing in his room anymore because of how weird it is lately.  I called the vet considering putting him down but it would cost $200. So I'm going to skip through all of the judge mental posts calling me a murderer xD lol and what not but can anyone give me a clue of a humane way to put down this guinea pig. I'm up for anything creative I just don't want him to suffer at all.  I literally paid like $30 for him along time ago and he keeps getting sick and I feel like he's just suffering and I'm not willing to spend hundreds of dollars at the vet for him when he's probably near end-of-life anyway. Some may say to just let nature run its course and let him die on his own but I do believe he is suffering and I just am wanting to put him down. So someone help me please this is hard enough as it is. I won't even do it my husband is going to have to I can handle it. He suggested rat poison but  I said NO because it's supposedly a pretty brutal death.