It's fine, but you're asking how good your sons BP performance is.
Better to measure exit velo off a tee. At showcases it is done off a tee, so the ball is stationary and everybody is on an even playing field. That will give you the most accurate representation of how hard your kid hits the ball. Can't have the speed of the pitch influence those numbers.
As for the numbers, it's good to see he is capable of driving the ball 300 ft, but I'm not sure how impressive that is off 55mph. Keep working, sounds like he's probably above most freshman, he'll be fine.