The only way you will know if his kidneys are compromised is via blood test. You need to draw blood and have the lab run his values against rat values. If his BUN, Creatinine, or Phos are high, then he is suffering from renal failure and normally should be put on an IV flush. At the same time, they can become anemic, and their thyroid can be thrown off, so not only doing CBC, but a SMA, a glucose panel and thyroid will give a more complete picture of his health. Also it is normal that if their blood count is low, they can be transfused with the blood of another squirrel, but that can only be done by vets with the proper equipment. Just continue to keep him warm because the more fluid that runs through his body, the colder he will be.
If continuing to sub-q and, once he is hydrated, you do not notice him urinating, a good diuretic is calcium. Do not use potassium because dosage is intricate. So you may want to supplement calcium as well.
It is good news to hear that he was up this morning and more awake. He is thin, so even though you are giving him the goat milk formula, don't forget about the vanilla Ensure Plus. Either he will hate it or he will love it and it will put weight on him faster than any formula on the market.
It also looked like he may have some hair loss on his side, which could be caused by low calcium levels if not caused by repeated rubbing against an object such as the entrance to a nest box, etc. The calcium "may" help with this as well.
Hoping to hear more good news later today! Big Hugs and Big Kisses to Chuck!