I can completely understand your worry. My oldest has had to take pain medications on a very limited basis and he never even finishes the bottles.
Also odd as it may seem, back when I had my hysterectomy they gave me a boatload of percocets even though I was already on a long acting pain medication twice a day. I had no issue not taking more than I needed. Though let me tell you, if you dont keep ahead of the pain it is twice as hard to get it under control. Stupid me thought I was superwoman when I had my knees scoped and thought my long acting medications would handle the pain...after all the knee was numb! I didnt take the short acting pain medications when I got home before going to sleep and I woke up in the middle of the night thinking my knee was exploding. It took me about 8 hours to get the pain back under control. Taught me!
I hope your difficult child is feeling a little better today but remember to take medications at the time said instead of waiting until he is really miserable. He will take less of them and get better faster.