I should think that assessing him while he is on medication might skew the diagnosis. Have you tried behavioral therapy instead of medication? ADD medication can have nasty side effects, some of which are as yet unknown.
To answer your question specifically, blood pressure for a 13 year old, according to national institutes of health, averages about 108/62.
But blood pressure varies by all kinds of different things (time of day, activities, etc.)
Your doctor should /must be able to tell you why his blood pressure and ADHD meds are linked. It is important for you to know that. I am rather curious myself.
Rest of your post is confusing. Sounds like they are not assessing him for Autism? Who suggested that he be tested. And maybe more importantly what kind of doctors is he seeing? And it sounds like he is seeing two different ones. Do any of them have any psychological background? In fact, have any of them given you any ideas on how to help your son, or do they just hand out the meds? I can understand your concern.
Someone may be able to here but I also suggest you check out the autism forum for further help,good luck