Dad wrote:

The additional activity that we have looked at are martial arts (Tae
Kwan do) and music lessons.

Just my bias, but at this stage I would add music/arts
or some kind of service sort of thing (scouting, etc.) rather
than adding another physical activity when he's already got
two physical activities. Since this is a good age for trying
things out, I wouldn't want to get him too boxed in yet.
If you elect music, however, be careful that whatever you
choose is really developmentally appropriate and that the
teacher is experienced with kids his age.

Best wishes,
Ericka