My Son Is Taking Oxycotin 40's, 4 A Day. I

My son is taking oxycotin 40's, 4 a day. I keep the pills, otherwise he abuses them. Is there a,placebo pill I can give him in place of 1"or 2 of them? I know he can't just stop them altogether. He has gone through the symptoms of withdrawal before when he has run out. I just don't feel his pain is bad enough to warrant the dosage he is given.