I recently adopted a kitten from the local animal shelter. He was only 4 weeks old, and very tiny.?

Once I got him home he started sneezing and had a watery eye. I took him to the vet and they said he was developing an upper respiratory infection. They sent me home with clavimox, l-lysine, and terramycin ointment for his eye. It has been getting worse, and now one of his eyes is swollen, his third eyelid is red and inflamed, and there is a lot of discharge. He is very congested and you can hear him breathing. He sounds terrible. He doesn't even weigh a pound so i'm very worried for the little guy. He has been to the vet 3 times now, and they said to just keep doing what I have been and that's all I can really do. They didn't prescribe a decongestant, so I was wondering if there was anything I could try to help him breathe a little better. I am feeding him A/D through a syringe as well as his water. I have been wiping his eye out every few hours and putting the ointment in it, but it doesn't seem to be helping yet. It has almost been a week. Every time my husband or I take a shower we bring him in there for the steam, and tonight I am going to buy a small humidifier for him. He sleeps in a heated blanket, and he is not playful at all. Today he started meowing more than normal. I was just wondering if anyone had any advice on what I can do to make him more comfortable until he hopefully gets over his infection/virus. He has no developed pneumonia yet, and he is going to the bathroom still. Any advice would be great! Thankyou! I just want my little baby to get better!