There can be no definitive answer to your question regarding a relationship between the administration of short term, high dose systemic corticosteroids and your son’s shoulder injury. That is to say, that the medical literature does not address this in a systematic fashion. See the following from, Clin J Sport Med 2005;15:370)
Conclusions: This critical review reveals that the existing medical
literature does not provide precise estimates for complication rates
following the therapeutic use of injected or systemic corticosteroids
in the treatment of athletic injuries. Tendon and fascial ruptures are
often reported complications of injected corticosteroids, whereas
tibial stress fractures and multifocal osteonecrosis were described
with systemic corticosteroids.
Long term steroid therapy can cause muscle weakness and atrophy but this is quite unlikely with 3 weeks of therapy. This is important, with long term therapy, because of the role of muscles in protecting joints, but just the same there are not definitive reports of injury secondary to the steroids and, in any event the therapy was appropriate for treatment of the Erythema Multiforme (EM).
Steroids can cause skin fragility, even when given for relatively short periods of time. This or tape allergy could have caused his skin to tear loose.
From your description, I suspect that your son has had what is called, Erythema Multiforme Minor. This can be associated with autoimmune diseases, but frequently is not.
If the EM recurs, you should seek consultation with a Rheumatologist, or even without recurrence you might want to do this, for your and your son’s peace of mind.
Good luck.
Thank you so very much for your informative and timely response to my questions regarding my son and his recent diagnosis of Erythema Multiforme. I REALLY appreciate the information you have shared.