What Disease?

My friend's kitten is sick at about 6 weeks old.
He was skinny when he got him and is now fatigue and breathes heavily.
My friend took him to the vet a couple of weeks ago and the vet gave him antibiotics. The kitten got better but he says now its back again.
He cant take the kitten to the vet due to it being Columbus day.
Any idea on what this could be? The kitten is moving now but he says not much. Its also not constipated as it poops alot everyday.

Emergency vet clinic asap.

It's possible the antibiotics did not get rid of the virus/infection, and the kitty needs another dose? Did your friend try calling the vet even though it is Columbus Day? Even if closed, there might be an emergency number to call.

What did the vet say the kitten had when he/she was seen and given the medication?

This is an emergency and the kitten needs to go to the nearest 24 hour clinic ASAP.

The kitten is doing better now. They most likely took it to the vet today so ill ask tomorrow.

Well the sick kitten, whose name was Tiny, died yesterday.
The doctor doesnt know why... -_-
She was only 7-8 weeks old.
He was the runt and the smallest one of the litter.
Out of the 6 we rescued one didnt make it

Aww. I'm so sorry!

Poor little bit.