My son is like yours AND we just got a diagnosis of PANDAS. Add aggression to the mix of the symptoms you've described ... as well as severe speech apraxia ... and that's my son in a nutshell.
Our dear child is on Tenex as well as Risperdal (0.5mg in pm) and Prozac (2ml in am). He has had severe bowel issues since he was 4 weeks old. We found he was allergic (confirmed by allergist) to soy, dairy, yeast, eggs, and corn. Removing those items from his diet helped the constipation somewhat, but only after we added daily probiotics (we use Pro-Bio Gold from Kirkman Labs) to his diet did we see daily BMs. His BMs are occasionally nasty (messy, large), but at least they occur daily.
Our son has ABA, which has significantly improved his ability to learn and communicate and function (of course, now the school wants to discontinue paying for it ... the only approach that has ever helped him). My son is 8 yrs old and we were relieved to get the PANDAS diagnosis, but we've only had the dx for a month and I don't think we're addressing it as aggressively as we could be.