acne is NOT caused by diet or washing or anything but the natural balance of hormones in the person's body. some things (like eating well, hydrating, staying away from irritants) can help minimize breakouts, but nothing but hormone therapy will STOP the body from producing excess sebum that causes acne. face washes don't really clear up acne, other than perhaps helping the skin to heal faster. the fewer irritants he comes in contact with, the better, so simple ingredients and non-abrasive materials are your best bet. not picking or poking at his acne will also help.
if this becomes a serious problem that affects his self esteem and is causing deep scarring, there are therapies that will help, but they are not something to use lightly. it's a bit easier for girls, since often the right bcp will balance the skin out.