Officers from one jurisdiction can issue a citation based on observations made by officers from another jurisdiction. Any other result would mean that the old fasioned notion of crossing the county line (or state line or municipal line) to get away from the law, which makes for great drama in the movies and TV, was actually true. Also, in this particular case, the law on which the citation is based is the state law, not a local municipal law, and so it makes no difference which officers make the stop.
Finally, and I know for certain this is not the answer that you want, but your son's duty to drive responsibility is your son's obligation. Anyone old enough to operate a motor vehicle on public roads must be responsible for their own actions.
I truly wish you the best.
This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Laws vary widely from state to state. You should rely only on the advice given to you during a personal consultation by a local attorney who is thoroughly familiar with state laws and the area of practice in which your concern lies.