RE: Start centerboard?
Yes, if you have a 48 cm fin, and there's enough water where you're sailing, give the larger fin a try.
Have him try the board without the center fin (put in the plug where the center fin goes).
Also have him try the mast base in the middle, and little behind the center, to see if that doesn't help with turning upwind.
Also, ensure that he's getting the mast well across in front of his front shoulder (well across past vertical and over to the other side so the board accelerates forward across the wind without turning upwind).
Hope this helps,