What's the main difference between Hapkido & Taekwondo and which art would be more practical to use when defending oneself on the streets ?
As well, I would like to hear from anyone what they think would be the best type of martial art to learn to for self-defense because I am interested in enrolling my son in an art.
OK.
Second things first:
any of the traditional MAs can be a completely effective MA for street defense, if that art is trained
for street defense. That means, trained not for point-scoring in sports competition, but trained in realistic one-on-one scenarios with one of the participants playing the part of a dangerously violent but formally untrained attacker, a description which corresponds to the vast majority of cases. Again: it is not the art per se, but how realistic the training is for violent street confrontations, with the skills of that art exploited to inflict maximum, possibly permanent damage upon a completely
noncompliant attacker. Karate, TKD, Jiujitsu of various kinds, Ninjutsu of many kinds, and on and on and on, can be ferociously successful in that capacity if the student is trained from early days to exploit the technical resources of the art
with that goal in mind.
Since that's true in general, it's true in the specific case of TKD/Hapkido. TKD is, basically, the Korean development of Japanese karate, augmented with certain elaborations of kicking techniques (some of which have in turn diffused back to Japanese karate). Hapkido uses locks, throws, pins and so on as main tactical weapons to a greater extent than TKD does (though TKD does use them to set up strikes). Hapkido is to TKD as Jiujitsu is to Karate: arts based on limb control, pins, throws and so on, though some strikes are also trained and used, vs. primarily striking arts with vertical grappling techs utilized to set up terminal strikes to weak points on the attacker's body. People often compare Hapkido in Korea to Aikido in Japan, but my impression is that the grappling/striking mix in HKD is more like traditional Jiujitsu than it is like Aikido. Again, both TKD and HKD, properly trained for street defense, can be savagely effective destructive combat arts. The crucial bit is the training philosophy of the school. If you want close-quarters self-defense primarily, you must make that clear to the instructors at the school you're considering. Check out several classes there to see just how much of that is part of the regular curriculum. You should get a good sense after a few visits of whether the SD aspect is a high enough priority at the school to yield technical competence for that purpose.
