Is the vet experienced with birds? Did they do any tests or suggest any treatments? If no to either question, he should have a follow-up exam with a bird-experienced vet. They may be able to help him.
He is at the most dangerous age for stroke and cardiovascular disease. Did this start out with an acute episode or seizure? Did the vet discuss this possibility?
If his head is tilted, he may feel dizzy, disoriented nauseous and very frightened. The white material could be vomit. I have had pretty good luck with giving them valium injections for a few days to alleviate anxiety and nausea, and this in turn helps them eat and drink. If they are still at the vet, you might see if that is an option. Injections are easy to administer at home.
Once he is home, your job is to keep the bird warm, safe, quiet, and confined; and to provide adequate hydration and calories.
If he is sweet and prefers being held, you can wrap him loosely "burrito style" in a soft old wash cloth and hold him.
If you have to set him down, move the bird to a box or carrier with soft towels in the bottom, no perch, and food and water in low bowls that can be reached easily. Put the whole thing on a heating pad on low or medium. Check it frequently, no overheating allowed! Keep the unit partially covered, warm and quiet. White paper towels or white cloth towels will show the true color of the droppings. This is important, to monitor urine output and characteristics. Small animal/reptile boxes are great for this purpose.
The bird, bowls and unit must be kept very clean.
Here are some helpful links:
If he is able to take warm water from a syringe or spoon, try and keep him hydrated. The best method is a small amount, often. 1 cc at a time is a lot, and if you can do this 4-5 times an hour that would help hydrate and dialyze. They can hydrate from oral fluids almost as quickly as IV if the GI is functioning properly. If he will do that, you can alternate with warm sugar water or unflavored pedialyte. For food, any pet store will have Exact baby bird formula, which goes through a syringe well when mixed according to instruction. Cream of Wheat, oatmeal, warm brown rice are ok to offer in small amounts and even tilted he should be able to prehend and swallow these. You can try mashing banana with the formula to make it more palatable. You can offer warm pancakes, cornbread, grapes, melon, greens in addition to normal food.
It is important that the cause of renal failure, if that is what is going on, be determined. Some causes for remnal failure respond to treatment and the bird can recover; some do not. There may be some treatable reasons, or at minimum the prognosis and quality of life can be evaluated.
Am I apologize, my phone coverage is poor and internet is the best means of communication.