M.S.
Go to www.drugs.com and look up Amoxicillin side effects. The drug also can cause fever, sore throat, vomiting and diarrhea, just to name a few. The list is huge. If he has had several rounds of antibiotics, he can also develop thrush, (yeast in mouth, throat, g-I tract). It is possible he has adverse sensitivity to this class of meds, however the "cillin's" are usually prescribed for strep infections.
Sometimes the most well intentioned don't work and he may need some other form of medication.
Hope you get right treatment soon!