Given that your only objective signs involve muscle impairments, and the symptoms come on late in the day and with activity, I would focus on a detailed evaluation of muscle length, strength, especially endurance and also examine activation patterns with upper limb movements, including upper limb movements with load. Also, check relevant activation of postural muscles and strength, endurance of scapula thoracic muscles.

What sports, work is he participating in? This may give you additional ideas

Interested in hearing what you find. I would not expect such chronic symptoms to resolve with stretching alone.