My 23 Yr Old Son Is Taking An Antidepressant (Sertraline)

my 23 yr old son is taking an antidepressant (Sertraline) along with Tecta for stomach upset. He also takes Strattera for ADD. He has fell off is meds due to a new job he started and a crazy schedule. it's been about 3 weeks he's been off his meds. he is fine menatally except for irritability and since he started back on his pills he has had bad stomach pain. he quit again becasue it makes him sleepy during the day so he started to take his antidepressant at bed time again . what can he do for the stomach upset? He wants to quit taking them again because of hte pain. he was told he should start taking a lower dose of his Sertraline and increase his tecta to 2x per day (morning/night). he was also told he might have aquired a resistance to the tecta. is that possible?