What age is your son? Are you looking for an easy, intermediate, or advanced level solo? Unaccompanied, or with piano accompaniment?
In my playing days, I got a lot of mileage out of Mozart's Clarinet Concerto in A. There are three movements (Allegro, Adagio, and Rondo/Allegro) -- your son could pick the tempo most suited to him. There are a number of nice arrangements for solo performance, particularly those arranged by Himie Voxman.