It would be impossible for me to give more/better information than you vet, still I would be very disappointed in the vet if the same thing happened to me.. I would surely look for a different vet going forward, ask around and find a vet with good references.
How old is your puppy? If he is drinking water and eating with healthy looking bowel movement I'd let the sleepiness go for a while. But, keep an eye on him for additional symptoms/problems.
You mention other dogs, do they all pay together and has the puppy been accepted by the (presumed) older and bigger dogs? If he is put down by an over dominate playmate that could be the problem. I have seen this happen myself. In that case the little playmate, a goldendoodle grew up to be considerably larger than the older "brother" who still tries to dominate but when goldie gets tired of she just knocks him over/down.
Parvo is quite devastating to the gut lining. It quite often kills young puppies, and indeed adult dogs. So your pup is at an advantage already in that he has survived.
Fighting this takes a LOT out of the immune system too. Your little pup then got a dose of kennel cough when he was at his weakest. Poor little thing.
He needs to heal and fully recover and this takes quite a while for the gut lining to heal and for him to tolerate food.
It could be that he needs an easily-digestible "convalescent" diet for at least a week or two -maybe even more, as his convalescence has already been put back a bit by his fight with kennel cough.
He needs plenty of fluids as well. (plain water)
Can the vet help with suggestions about what to feed him during his recovery?
I don't know all there is to know about dog nutrition. But I would say that if you can start him on baby food (regular baby food) -make sure it's onion-free!!....scrambled eggs....rice....mashed potato...chicken breast...white fish. He might find it easier to digest if the food is put through a blender first.