Ritalin is a stimulant and can increase anxiety and intolerance to pain. That could be the effect.
I'm not a doctor, but my input that I could include is that I'm currently on Lexapro and Concerta (ADD / ADHD) lexapro can cause suicidal tendencies in people with mania bipolar disorder. Oxycodine if that's what you ment is used as a downer which could also cause depression but can cause major irritation to small problems. I looked on a drug interaction checker for Ritalin, Lexapro and Oxycodine and it doesn't really have too much of an interaction. My guess is he has some sort of bipolar disorder or manic depression/ major depressive.
Try getting him put on some sort of mood stabalizer for the time being and give it 2 weeks and see if there's some sort of difference.
Good luck,
Steve