To answer your question. A trombone has a standard range which almost every intermediate musician can play, but then its possible to get further notes in the upper register by lipping them (same with sax). You can't extend the register down below the lowest note on either instrument. As a party piece I knew a Euphonium player that played chords (simultaneously). Its all about the embouchure.
I play both sax and trombone to a degree and I would not say that the trombone is harder, you can't really compare fairly. At beginner level, the trombone only has seven 'positions' whereas the fingering for the sax is complex, not intuitive. For both instruments you need to develop a 'lip' or embrouchure, its a bit easier on the sax, but not that much, its really something that develops over years and years. I find on brass instruments the lip deteriorates quicker (even a week away makes a difference).
If you can sure your son does a few minutes each day, it will keep those muscles developinghttp://www.alexanderpublishing.com/Images/Spectrotone%20Chart/Spectrotone70thAnEd485x364-72dpi.jpg