I think Shawn did a great job of answering this question along with his comments. If it were me, I would throw as many assessments at him as possible. It’s important to keep in mind that a 14 year old is not fully developed.
Having said that, the young man may be able to do most anything if he is in good shape.
I would start off by doing some flexibility and functional movement assessments. Make sure he’s getting around well with his own weight before moving on to more difficult or loaded exercises. If he’s an athlete, then he’ll most likely benefit more from training “sport specific.” A football player needs different routines than a track star, although there is always overlap, or at least there should be!
Even further than that, what position is he playing on the team? Depending on the demands of his position, he’ll need a specific schedule to increase agility, speed, power, endurance, etc. All of these are important to any athlete, but as far as answering your question on what assessments to perform, more information is needed to give a very specific opinion here.
For instance, if this athlete is doing track, then he will not really benefit from an assessment of his maximum lifts. They can be used as a reference, but probably won’t serve to be of much benefit in organizing a program for him.