I would recommend that the diet be heavily, if not exclusively KMR. Also make sure that the purees do not have any onion or garlic in them. While onion ingestion in healthy adults does not often lead to clinical signs, in small kittens it can be a big deal. Also using an eye dropper to help supplement with fresh water can help out. If the kitten is deteriorating, then a veterinarian visit for examination and possible antibiotics and subcutaneous fluids may be beneficial. If the kitten is having problems passing stool, then 1) keeping fresh water intake up and monitoring for dehydration can help along with 2) warm water baths can help relieve constipation. Just make sure the water is not too hot and that he does not get too cold afterwards. Hope that helps.
It was wonderful of you to save him. He is a lucky guy.