Dearest Sir Robert de Mere,
Thank you for sending a bill of exchange in advance, and for your understanding - if I gave out advice for free, I'd very quickly run myself out of business.
You wrote that your son went missing while on a voyage - and that near the place he was last seen there now lurks a nekker who wears a tattered shirt with your son's monogram sewed on it. You asked if it is possible that this nekker was your son, transformed by some spell.
After consulting the relevant literature, it pains me to inform you that your hypothesis is highly improbable, and there exists a much simpler and more likely explanation. Contrary to popular belief, nekkers are a sapient species. Like forest trolls, they will at times decorate themselves with pigment or scraps of human garments. One can with some confidence surmise that this nekker killed your son, tore up his shirt, and then put on it or some fragment thereof.
While that is most likely the case, one cannot be certain. Though I have not heard of any curse capable of turning a man into a nekker, I cannot rule out the possibility of its existence. In order to find out if the above-mentioned nekker truly is a creature born of a curse, the tattered shirt it wears must be taken from it, cut into shreds and burned. The shirt's ashes can then be mixed with dimeritium dust. If the resulting substance changes color, it will confirm your hypothesis.
If the experiment I described returns a positive result, please write me. I would gladly undertake further examinations meant to find a way to life this curse - for additional payment, of course.
Klaus Kellerman, Magister Artibus Oxoniensis