So, what exactly did/do you use for deworming?
If I understand you right, he was already weak/ill when you brought him into your flock?
For how long did you quarantine him if at all prior to adding him?
I know this from my own experience.
The first time I had to do it, left me crying and sad. This was decades ago, but I still remember it.
Hopefully he will pull through.
Did you check him for lice and mites? When did you last deworm him?
This exactly.
Get over yourself and help the poor guy out of his misery. Or ask a vet/neighbor/friend/family to help do it.
This is one of the important things when keeping animals:
Don't let them suffer!