I know that you asked about 75 grain broadheads but I would consider stepping up a little in weight to an 85 grain broadhead. There is a heck of a lot better selection in 85 grains than there are 75 grains. He isn't going to notice much difference in trajectory at all by adding 10 grains of weight. I

If it were my kid though I would have him shooting a 100 grain broadhead. He will have a higher FOC and should have a little better penetration. I doubt that you are going to have to worry about the spine of the arrow because at only 37 pounds of draw weight he is likely over spined quite a bit any way. Of course he will have the best selection of broadheads available in 100 grains. I personally had my daughter shooting a 100 grains standard slick trick when she was only pulling 35 - 40 pounds. In fact, that is what she is still shooting at over 50 pounds.