Talk to the doctor about his symptoms. The Decadron certainly can be causing him to be sleepy, etc, but it could be a build up of fluid as was the case with one of the other responders to your message. Only diagnostic testing will tell for sure.
You say he is on alternative medicine - would be helpful to know what he is on, to see if that could be contributory to him not seeming to do so well. Everybody's body chemistry is different. Just as some prescription meds and chemotherapies are good for some people but not others, the same can be true for alternative treatments. If you are getting guidance from someone about this alternative treatment, let them know about these problems - they may have encountered them before and have answers for you.
Don't let the doctor give you any problems about investigating why he is so lethargic and unable to do things for himself. If you don't want to go back to the other treatment - that is your right. Some doctors will say that if a patient will not take conventional treatment there is nothing more they can do for them, but that is nonsense. Patient's and their families have a right to choose, and the medical establishment is not justified in denying care just because a patient has exercised that right to choose. Tell the doctor you expect him to keep working with you on this while respecting your wishes, and that if he does not you will report him to his employer and the medical board. If you don't get help from this doctor, or if he says there's nothing he can do, get a 2nd opinion.
If the doctor wants to do more tests, I would let him. If they say it is fluid pushing on the brain and they want to insert a shunt, I would think that is a reasonable solution.
Good luck, and the best of health to your son,
Sincerely, Tre