Keep him warm enough &in a draught free area. The steam therapy should help him breath more easily, if u have any olbus oil-leave some on some cloth/kitchen towel near his cage/hutch not within reach of him, this should also help to clear his airway.
My vet offers cheaper follow up appointments for 2nd or 3rd etc visits for the same thing, might be worth asking if your vet would consider doing this for you?
I think
@Adelle s fudge has a cocktail of different meds &frusol/diuretic too,
My ellie had it but I can't remember which other meds she was on at the same time.
DanDilions &celery have some lightly diuretic qualities if you have any available you could dry giving him these. (dont give him too much though as an excess could cause a tummy upset or bloat, which would complicate matters)
The fact that he is eating,putting on weight&pooping etc is a good sign.
Is his hay of a good quality? Just thinking if u have a dodgy batch of hay, this can cause respiratory problems-also bedding guineas on wood shavings/dust can make them wheeze too.
Check his hay hasn't gone mouldy &replace if it has.
One of my girls picked up a bad URI after a dodgy bag of hay. Took her ages to be rid of it.
Might be worth asking to extend the antibiotics for another week. 5days doesn't seem very long.
Healing vibes for your little man.
Speedy recovery. Xx