dexedrine and zoloft | ADHD Information

My almost 12 yr old son just started Zoloft...Dr said to start at 12.5mg and work up to 25mg.  He also takes 54mg of Concerta, 1MG of Tenex in the am and 2mg of Clonidine in the evening.  The Zoloft is for the Anxiety...which is terrible.  We tried Celexa and Paxil CR..neither of which helped much.  The DR feels Zoloft is going to be better...it's only be 2 days....I will let you know.  I am curious on other experiences with Zoloft.We will also start my son on 12.5 mg. of Zoloft and possibly work up to 25 mg. Gave him 1.5 mg. of guanfacine today and will work up to 2 mg. We will wait and see if we need to start the Zoloft. The doc said if we started the Zoloft, to take him off of the guanfacine.

Just got back from the doc and we talked about increasing my son's dexedrine dosage from 5 mg. 4x per day to 10 mg. 4x per day. We are going to look at increasing his a.m. dose of guanfacine from 1 mg. in the a.m. to 2 mg. in the a.m. to see the response. He also has mentioned taking him off of the guanfacine if we don't see any improvement and putting him on 25 mg. of Zoloft in the evening for anxiety. My son has ADHD and is also starting to have issues with his behavior at school. Wondering if anyone has had any success with increasing the dosage of a stimulant or putting their child on Zoloft? 

My son is 10 years old and has a significant delay in speech with a low IQ.