My Son Age 7 Was Prescribed Zythromax For A Sinus

My son age 7 was prescribed zythromax for a sinus infection 4 days ago. After day 2 he began complaining of leg pain. He has only had his first 2 doses because I stopped the medication after he began having pain. He has not had it in 2 days but is still in pain and having trouble walking. Should I give it another day before seeking treatment? Is this a possible side effect of zythromax?