B.C.
We used it and it was fine.
Actually for a few days after the sugery (this was 13 yrs ago so I don't know if it's changed any) they gave us a pink liquid pain med to use for the first 48 hrs and THEN we did liquid tylenol.
For the few days he'll need it for pain - I wouldn't worry about it - just follow doctors instructions.
Do NOT wait for pain before you give meds - you follow doctors schedule for the time period he specified.