He's a grown man. He can deal.
You owe him nothing. Yes, it's sad. You can't fix it, and it is definitely not worth even trying. You're probably like I am and you're sad about the loss of a possibility or sad about a Disney story in your head about love and forever.
If he's using drugs, you'll start finding things missing from your house and your wallet if you let him back in ever. He's told you who he is. He takes money from your kid and goes to use drugs.