The ENT referral is a good one b/c mouth breathing is NOT normal and will change the structure of the face. You can read about this my website: www.expressionsspeech.com
It's called Adenoid Faces and it's a real problem.
However, it could be the tongue thrust causing the mouth breathing since it has affected the teeth. Might want to look into referring to a myofunctional therapist as the sounds might not really be the problem, it just might be the tongue thrust. Some orthos will also insert an tongue crib to keep the tongue off of the teeth at rest, but he sounds like he might be too far past that point for it to be effective.
I would target more than one sound at a time since he is seen individually. I would focus on the 'sh' and maybe final /s/? Move from words to phrases to sentences after 90% accuracy at each level. Wish I could help more, but these kids are really hard to help!