Thanks so much for the answers to my questions.
First, let me say that black stools (aka melena) can be somewhat concerning since this usually reflects digested blood secondary to a bleed somewhere in the intestinal tract.
If he's becoming weak this could be secondary to anemia. It's a shame that you can't check his gums (although I certainly don't want you to become injured) since they might be on the paler side if this is occurring.
I'm not a huge fan of coincidence although I'm having a difficult time connecting an anal gland infection
with his current problems. However, it's possible that the infection triggered something like an auto-immune condition.
It's also possible that it's not related at all and something else is going on such as a tick disease or cancer I'm sad to say.
I've also seen idiosyncratic reactions to antibiotics of this sort although it's not common.
I'd love to be able to advise you to give him something over the counter which might help with this problem but it sounds like his condition is serious enough that a recheck would be advisable.
I realize that my answer may not be what you want to hear but I would be doing a disservice to both you and your dog if I were less than truthful and honest in my response to you. I hope you understand.
I also hope that this is helpful in understanding that there may be something fairly serious going on with him. Deb