when writing this sort of thing, have a look at the school's own prospectus to see what they think they offer.
School: we have a widely renowned economics department with particular emphasis on international trade systems..
Your son: as I take a great interest in international trade systems, I hope to make the most of your well-known economics teaching...
Well, you get the idea; don't cope the working too obviously, and stick to telling the truth, but remember they will be wanting him as much as he wants them.