I want a coworker to take personal leave until they can concentrate on work
Are you going to tell him?
I want to, but I'm afraid he would get too mad at me which means he wouldn't listen to me even if I told him
You should politely make a suggestion. If he's not ready to act, then at least the situation won't get any worse.
Actually, I think I'm going to just pour cold water on him like he did to me.
I assume that you are talking metaphorically, and that you mean that you are just going to bluntly tell your co-worker how you feel.  However, I have to remind you that you already think your co-worker won't listen.  In that case, being blunt with him will just make things worse, but if you politely suggest time off, that might help.