Like others have said it just depends. Can you still easily feel his ribs, but not see them? That is the ideal weight-feel the ribs, but not see them. You should also be able to see a nice tuck at the stomach.
Tysen was about 75 lbs at that age and getting 4 cups a day and it was a struggle to keep weight on him. Now, at 2 1/2, he is down to 3 cups a day.