My personal experience is that it really helps to learn both! It makes you a more versatile musician. Most music in high school is available in both clefs. If your son wants to play in a brass band though, staying with treble clef would help with that. My personal problem is that whenever I read bass clef now (because I switched from trumpet), I associate the bass clef fingering with the treble clef note name. For example I would still call an Eb an F. So teaching your son the right way is imperative so he can learn equally well. Flash cards would help with this. If you show him a note on the bass clef staff then have him press down the corresponding valves while saying the correct note name, that would be most effective.