I wasn't trying to get too detailed on that, concidering he hasn't had a proper history or physical, so I said maybe and used a vague general term. But here's the low-down
Spain-injury to a ligamentous structure
Strain-tear of muscle fiber
Tendinitis-tear of a muscle tendon
Tendinosis-microtrauma or dengeration without inflammation to the tendon.
Paratenonitis-inflammation of the paratendon sheath.
Key signs to differentiate these are location location location, pain with active resistance, pain with passive movemetns, pain only at end range, crepitus, creaking, snapping, popping, warmth, redness, swelling, point tenderness, onset of pain.
It's almost impossible to completely and accurately diagnose the difference, it's more of a histological differentiation you can see on a slide, or hopefully an MRI.
Either way, the treatments are very similiar.