I haven't read the other posts - so I'm sorry if I'm doubling up on comments - but that sounds like a classic anxiety disorder

Easily treated though - get him along to your GP for a referral to a counselor and they will help him with age appropriate coping techniques when his thoughts are getting the better of him.

It sounds like he is an extremely thoughtful young man, a very forward thinker (which is a fantastic trait to have) sounds as though he just needs to refocusing