Hi,
You don't say where in the arm the muscle is swollen. Above or below the elbow, front of back? Maybe you could ask your physio which muscle or group of muscles is affected? Is he right or left handed, in other words is he holding the plaster in his left hand or is he plastering with his left hand? I don't know the official name for it but the board that holds the plaster plus a full load of plaster can be heavy. Is the problem at the site where he used to inject himself, and if it is when did it start? Before he stopped the Heorin or shortly after? I know this seems like a lot of questions but it would help to suss out what the problem may be. Your son has had a load of tests you say. Has he therefore been referred on to an Orthopaedic specialist?
Personally I do not think exercise is the right way to go at the moment. If there is inflammation present then rest would seem to be more appropriate. My immediate thought on reading your post was that this could be a Chronic Compartment Syndrome. Most commonly people associate this with the lower leg where it is known as Shin Splints but it can be found in other places on the body including the arm. Other thoughts are Repetative Strain Injury or scar tissue build up at the site of injection.
If you would be happy to answer some of the questions above I would be in a better position to help. If you do not wish to do this on a public forum you could e-mail me mrsa-1@runbox.com I don't promise to give you the answer but may be able to help in someway. By the way I, too, am a Physiotherapist.
Regards
Gill