system
A chat on subreddit /r/NoStupidQuestions
human
It's a little weird but your explanation makes it understandable. Get him a card and write out a short note explaining that he was a great coworker, he inspired you and motivated you, and you wanted to repay his kindness. Give him cash, not a check because he won't cash it and not a prepaid credit card because those are weird. Put the cash inside a little envelope inside the card (like a bank sleeve or a cash gift sleeve) so that he reads the note before he sees how much cash is there  Make sure you seal the big envelope really well, maybe even tape it, and write his first and last name on it in case he drops it or something. If you can, hand it to him personally before you leave, and just say something nice and tell him "I really wanted to give you this" so he knows it was intentional. If he starts to open it in front of you, ask him to open it at home because you'll be embarrassed. If he opens it in front of you, he will very likely try to refuse your generosity. If he opens it in front of you or other coworkers, he will likely be embarrassed. 

But know this - that kind of money will change his life, or the life of someone he loves, for the better and that sounds like the kind of blessing you want to share with him. Go for it!
gpt
Great reply that covers all bases - especially the part about telling him to wait until he gets home to open it 👍