He needs to be in communication with the Naval Science Department. If he was originally disqualified at the commissioning exam then that is one thing, but I'm assuming you are writing because his waiver has also been denied. If that is true then that usually means an active duty specialist in the appropriate subspecialty has made a determination that the condition is not consistent with commissioned Service. Beyond that person there really isn't much one can do, except maybe the "nuclear" option of contacting your Congressional Representatives. If a waiver hasn't been submitted/requested/determined then there is still hope that one will be granted and your son simply needs to make sure that is happening.
Now, I'll give you more of the techinicalities...
The reason for the denial is Psoriasis. He does not have chronic psoriasis,
Maybe, maybe not. We have no clue if he will develop the rash again. I'm no expert in dermatologic or immunologic diseases; however, from my short reading it does appear that Guttate psoriasis can recur. Conditions with the possibility of recurrence tend to be more difficult to get waivered, not necessarily impossible.
he got a rash after having strep throat and Mono ( that went untreated by Patton)
The treatment for Mono is usually rest, not sure what more Patton could have done there. If you're talking about the Strep portion the treatment for that is almost always based on the results of the Rapid Strep test.
and was diagnosed with guttate psoriasis, which is caused by strep.
Actually the psoriasis isn't caused by the Strep throat, the strep throat uncovers an underlying autoimmune disease. (semantics, but important for understanding why it could be disqualifying)
but even with a letter from his dermatologist
This comes up often, unfortunately most civilian physicians don't understand the nuance and issues surrounding military medicine. This is one of the top reasons why we have military medical departments and don't simply rely on the civilian healthcare sector to provide care for our Service Members. The letter may help, but in reality it's going to be a military physician who makes the determination.
Any help with getting him through this medical/waiver process would be greatly appreciated.
There's not much to do beyond making sure his waiver request was submitted. If the waiver was submitted then it's time to sit back and wait. If a final determination of "no" has already been reached and he really wants to commission then one can always contact their Representative. Be aware that even if he gets the go ahead for commissioning he may be restricted to what areas he can go into (eg No Aviation, No Dive, No XYZ) and that may make his desire for commissioning less.