19 yr old with four concussions and mctd. can one it both diagnosis cause changes in school and decision making?

Yes: Cerebral concussions are brain injuries and if severe enough can have cumulative effects. See a neurologist or neurosurgeon who can recommend testing to answer your question. Mctd is an autoimmune disease similar to rheumatoid arthritis. It is not likely to have anything to do with your son's difficulties.

See below: Prednisone can alter one's mood which can affect decision making especially if it makes one manic. However; concussion do affect brain function.

CONCUSSION: Concussion causes violent shaking of the brain disrupting normal brain chemistry and function. Cognitive dysfunction is fairly common after concussive injury, often manifesting as difficulty problem-solving, concentrating & activities involving visual pursuit such as reading books & computer screens. Recovery should be closely monitored by someone specializing in concussion.