triple bypass severe pain during recovery

Hello,
My dad had a triple bypass 3 weeks ago followed by some complications (a mild heart attack, temporary emphysema, fluid in the lungs) due to this he had to be in ICU for about 5 days and but was released from the hospital about a 9 days after surgery. My dads mind seems all there, when he talks he is like his usual self, but he seems to be in much more pain than is normal. His pain comes on suddenly and he basically screams because of how much it hurts. This seems to worsen after he coughs. He isn't very active yet, as he is still in severe pain. Three days ago we had to take him to the ER because he had a bit of a fever and it turns out he has an abcess (abscess) at the bottom of the chest wound, which they had to open about 3 inches so the secretions would come out. It will remain open until it helas itself and secretions stop. He was prescribed oxycodone (5mg) and we give it to him every 4 hours but it doesn't not seem to be enough. The doctor said we could even give him 2 at a time but taking that much oxycodone scares him. Can someone tell me how much is too much? Right now we give him one oxy, then 2 tylenol 2 hours later. So basically he's getting oxy or tylenol every two hours, yet he still seems to be in so much pain