Contrary to what a poster said earlier, this is not a UCMJ issue. Rather, it's a matter of contract law and requires understanding of both the AROTC scholarship contract form and AR 145-1, which governs AROTC.
My reading of the AROTC scholarship contract tells me that not maintaining status as a full-time student is a breach of contract and therefore grounds for disenrollment. However, I couldn't find any requirement for a specific number of minimum credit hours for a single semester. Rather, the definition of a full-time student in the scholarship contract is:
"A full-time student is defined as one enrolled in sufficient academic courses to obtain sophomore, junior and senior status at the end of each one-academic-year increment for the duration of the scholarship. This includes the required Army ROTC courses..."
This above language is more specific than anything I found in AR 145-1. Please not that I am not attorney, nor do I play one on SAF. (But I am proud to say I am a high school graduate.)
Did he have enough credits to meet the above definition even with the drops? Did the credits for his summer school courses factor in meeting the minimums? Is summer school considered an extension of the academic year? Did his GPA fall below the required level? Most important, did he have any communication or understanding, written or otherwise, with his PMS or MS3 adviser about these matters?
Separate from these academic matters, there are also ongoing requirements to notify the PMS of any medical issues, such as his injury, which could result in suspension of the scholarship.
I don't know whether he needs an attorney or not. It would be preferable if he could work this out himself. However, he needs to understand his obligations and rights under the contract and AR 145-1, some of which are gray. To complicate matters, there are a number of alternative routes, including Leave of Absence, disenrollment, and re-enrollment. So if this is above his head (and it would be for most 21 year olds) then he may need a knowledgeable adviser or other representation.
Good luck to your son in getting back on the road toward commissioning.