TL/DR: Pro players estimate that 10-40% of players are using HGH (to say nothing of other PEDs). That it's very difficult to test positive for HGH (and weed). Almost sounds like you'd have to be an idiot to fail one of these tests... Just like my friend from SMU said.

https://bleacherreport.com/articles/2602986-players-say-the-nfl-has-an-hgh-problem-even-if-peyton-manning-isnt-part-of-it

Additionally, your son's summer experience is so recent and so limited that you and he know practically nothing about the strength and conditioning culture at that school. OF COURSE the coaches would sit him down and tell him to get clean - they deal with this all the time, with players trying to pack on weight between hs and college. OF COURSE they'd warn him about supplements and tell him not to take anything right now - they know that he'll be tested for PEDs and drugs shortly after he shows up. But subsequent tests will be few and far between (and he'll almost certainly be given advance notice before he's tested - enough time to clean up and try and flush his system/dilute his sample should he need to do so).