All true, all true! With resting position, it can give clues but observation of dynamic function is better. I know because i have poor starting positions but functionally not that bad.

Lets face it, if the guy is an amateur, he is probably doing too many chest press/pecs exercises and not enough "back" and shoulder exercises to balance it all out.

In addition to the T/S and nervous system as neurospast has pointed out, consider also his cervical multifidus and longus coli - the easiest test for this is to lie supine, hold the articular pillar on both sides (anywhere you want, you will eventually cover all levels), then get the patient to flex 90deg to the ceiling then horizontally extend so they end up in 90deg abd. Throughout the movement, there should not be any lateral displacement of the cervical vertebrae to the moving arm side. If there is, cue them to grow 1mm higher and see if that stabilises the vertebra. If it does, then i would focus on motor control and retraining his cervical stabilisers.

Have fun! Sounds interesting.