For ear, nose and throat infections, if the infection is moderate to severe, a pediatric dose can be as high as 875 mg every 12 hours or 500 mg
every 8 hours. You are somewhere in between so it should be okay.
However, amoxil does indeed cause diarrhea in not only children but also adults. I would call your doctor about this. There may be an alternative antibiotic for your son that won't wreak havoc with his intestinal system and be just as effective.

zuzu xx