I like the idea of teaching discipline and respect and self-control, and a boost in his self-esteem would be wonderful.
Sounds like you want something that is practical, and you also want him to learn discipline and respect. Obviously everyone would want something that is good (ideally the best style), but then there is also the time investment required - which tends to be longer for the better styles, especially when it comes to something that embodies the non-violent ideals that includes self discipline.
Depending on what is available in your area, you might want to compromise and just go with something that is pretty practical with a teacher that's pretty much just a regular guy, neither a violent egomaniac nor a sage like figure such as Mr. Miagi or Kwai Chang Kane from Kung Fu. As much as I like to see kids learn the virtues and non-violence and all that, (hey, that has nothing to do with how articulate you are ) we have to recognize that people who are examples in this regard are relatively rare, they may not be martial arts instructors, and frankly a lot of it comes with maturation anyway.
The only thing is to not let martial arts training become a hinderance in this development, and a good rule of thumb would be where the development of self-esteem makes it so you don't need to fight, rather than just being confident about fighting - perhaps even overconfident. You just have nothing to prove. A lot of techniques and mental attitude comes down to redirecting the energy another person throws at you, but there is another 'level' where you don't even redirect it - it is literally neutralized, turns into 0. So in regards to self esteem, it isn't like it is so high, you feel so confident that the insults thrown against you is weak in comparison, it doesn't fall in the same category as force against force or even a skillful redirection. It just does not touch you, so the energy dissipates. It would kind of be like the master just stepping out of the way, so the attacker never even lands a punch in the first place. The same applies to mental and social interactions.
So these are things to keep in mind for the long haul anyway, and it would be nice if you can work with a MA teacher who can reinforce it in some way or atleast does not send a contradictory message either by word or action. As long as they don't do that and are a decent enough MA instructor then you'd be doing okay.