Slow Responder to Isotretinoin? When Can We Expect Results?

My son is 15 and 165 lbs with moderate acne. He took isotretinoin 40 mg first month, 80 mg second month, 100 third month. His 3rd month will be finished in 5 days. His face still looks worse than before he started taking the drug. He is quite frustrated. He had a very positive attitude and had been very patient, but it has become very hard to put up with all the side effects and still see so much acne breakouts. Please let me know if this is common and when he should expect results.