Our LLMD recommended Jarro-Dophalus EPS. (Don't know why that brand). I have been able to get it in our grocery store and at a local vitamin store. It is in the refrigerated section for the vitamins.
In the past I have used all types of brands and had no problems. My son is 11 also and has been off and on abx for the last 2 1/2 years. Thankfully he has had not negative stomach side effects for them.
Good luck.