No kill shelters to take a stray?

We found a kitten yesterday in a ditch. Took him to the clinic and got him vaccinated, dewormed, and a month of flea protection. Currently trying to keep the little guy alive. Have a heating pad on low in his bed, water, kitten milk, and wet kitten food. Anyone have additional advice to keep him alive? Anything else I should be doing? I took in a stray before but she wasn’t this young or as underweight. This little guy is so small so I am taking all the advice possible to keep him alive until we find a safe shelter or someone to take him. Thank you.