That is wonderful that you took him in. You are right, going back to Momma right now, probably would be an option, if the owners were that ready to home them.
Put a bowl of water down, and canned food diluted with either warm water or the KMR that you have. He should begin starting on solid foods now, but may not eat dry kibble for a few more weeks, unless it is moist. If he hasn't ever started solids yet, it may take him a few days to get used to it, but within a few days he will be an expert at eating the canned food. You will want a good quality food as well to start him with.
You may want him to stay in one room for now, such as your bedroom, with his litter, food, water, and toys, since he is young and just came to you, it will help him to find everything.
Check his stool, he may need to be wormed soon, and his first set of vaccinations will be coming up as well.
It sounds like you are going to give him a very loving home, and I hope that everything goes well.
We would love to see pictures soon. Does he have a name yet?