To feed him the potion you first need to find the chocolate frog. Go back to Harry's room and put the skull in the cabinet and close it. Open the top cabinet and you'll find the chocolate frog. Use the ladle on the chocolate frog then feed it to Peter Petipuff.
It's a great game but there are a few things that need to be fixed.
1. Armour should be percentage based and not flat as it can become very over powered.
2. You walk much faster diagonally rather than straight.
Thank you :)