Donna,
Wow, 6' tall at age 14. That's going to make him unique from his peers by itself, I would imagine. Yes, it's true that boys go through the teen growing spurt, and have to readjust to their muscle coordination, etc. I can only offer an addition of agreement with the other insightful advice already given here. The others have made excellent points that I would have told you and your son too.
Always consult the instructor. If the instructor is experienced and wise enough, he will be able to determine if there are other issues, which I too, suspect there might be. A student will seldom stop training, if they enjoy it so much, simply because of the "possibility" that they might hurt someone. Especially if that has not yet happened. In my opinion, it would be wise to press the issue until the full problem is revealed (if there is more), then it can be delt with. (Some things to consider are the rank issue between you and him, although he might deny it, or his awkward feelings of being a tall teen, any concern he might have about becoming a class leader or assistant instructor as a high rank, financial issues, new girlfriend, or other conflicting interests, any conflicts or comments between peers at Martial Art class, or outside friends making fun of his training).
If your son's concern for safety is the only issue, I would tend to suggest that you insist he attends regular classes under his instructor's guidance. Only through continuous training will he be able to overcome the very issue about which he says he is concerned. There are ways for him to train without risk to opponents, and he should be made aware of this. If his instructor is informed of the problem, training on bags, and hand targets, plus slow motion sparring will help during the transition. Also, I often train in sparring without throwing a single technique. The idea is to learn distancing, and how to avoid being hit by evading while only blocking if necessary.
There are many skills, and benefits to be gained by your son's continued participation. As a Brown Belt, he should be aware of this fact, if not, then you and his instructor must help guide him to see this. He will not benefit from home training, without the guidance of a senior instructor, as he would at the dojo. He should be strongly encouraged to attend classes, and improve skills other than sparring. I know parents want to be understanding, and give their kids some space, but don't go too easy on him. Without being "mean," a little assertive, loving, parenting push might be appreciated down the line someday.
That is what I would recommend to my own students, and their parents.
Good luck!
CM D. J. Eisenhart
