Um, your son is taking, IMHO, for a replacement dose, quite a large dose.

Second, cortef is one part of what needs to be replaced but with low BP, then you need minercorticosteroids - florinef. Strange that the endo has not figured that out.

Addison's is a salt-wasting disease. The low BP is showing that he is not managing his fluids. He needs salt tablets at the minimum - and some florinef added. Cortef has a tiny bit but not enough for hardly anyone.

I have no adrenals at all... and my replacement dose is only 22.5 (split 3 ways) and my docs say I would be on 20 if only they were not trying to suppress my ACTH. Please talk to your doc (or consider a new one considering this one does not know much about addison's!) about lowering the dose - gradully of course so he does not get into trouble. Otherwise he will have long term effects from the steroids. Also look into salt tablets.

30 mg of cortef is usually the maximum dose of cortef that doctors will allow unless you are stress dosing for an illness, I have been on 30 for about a year, and am about to start tapering to 20 as soon as they get my low blood pressure under control. The ,most I have ever taken is 60mg in one day due to oral surgery on my teeth.
The elevated blood pressure is a concern and is not safe. I would consult your doctor and begin searching for one who is familiar with how to treat this disease. Your son is probably going to need florinef as rumpled said to supplement what he is taking.
I hope he is better soon.