Glad to her the little fella made it through the night.
Sea salt is regular salt that still has all the important trace minerals. White table salt has been 'purified' so much that it is basically dead and is not healthy. Sea Salt is very healthy and in small quantities helps recovery from dehydration and heat prostration.
Keep offering fluids and following whatever instructions the vet gave. Let the little fella rest as much as possible in a quiet area.
As far as not urinating, he may have been so dehydrated that the fluids were needed to rehydrate the cells. However, if he still has problems urinating, talk to the vet about it.