Re: Going down hill fast.
You should visit a functional or integrative medicine doctor or a reputable naturopath ASAP for a second opinion. To me, he may have an autoimmune disease. These can be very difficult to diagnose, and mainstream medicine has a terrible track record of misdiagnosing them. He may also be dealing with an infection, such as a parasite. Candida is not so much a root cause as an opportunistic fungus. Everyone has it but it will overgrow if you have a weakened immune system. Destroying the candida without strengthening the immune system is a waste of time. It will just come back.
I highly recommend you research the GAPS diet, which is based on SCD but addresses some of SCD's shortcomings. The book is hard to find but you can get a lot of information online. You start him on broths, overcooked vegetable and boiled meats and work up from there. Also, you would include probiotic rich foods to build up the good gut bacteria. You also need to avoid all gut irritating foods, such as grains (especially gluten), beans, nuts and seeds. These are too difficult for him to digest. The gluten will break down the gut lining, a big no-no. Please look up the diet. It sounds like exactly what you need to make sure he is able to digest his food and get the nutrients he desperately needs right now.
Avoid
Antibiotics as much as possible. They destroy the immune system and gut lining and wipe out all of the good bacteria along with the bad. The anti-fungals may also be contributing directly to how he is feeling. He may be experiencing a high level of die off, which makes people feel terrible.
However, do not come off of any medication unless under the advice of a doctor. Your son sounds like he is in a very precarious position. You need to find someone who will help you to heal his body.
Good luck!