Hey MDF it's never too late! The benefits can be felt and seen almost immediately and if that doesn't convince you I don't know what will!
Rindy- I've just moved to Vancouver and I might be moving again so soon. Tai Chi will have to be put on hold... I may be moving but CelticRadio is not. So I'll let you know as soon as I get the chance to try it!
On the bright side, a friend of mine has been in contact with a trusted club and they'd be willing to find me lodging and cheap fare if ever I'm in their neck of the woods. I just might be. I practice CanRyu Jitsu and I'm beginning to find out that we seem to be a close knit community(for those who really practice it).
Arizona- Celtic's advice is sound. You mentioned that your son is tall. If there is one thing I learned quickly in martial arts..regardless of the chosen discipline is that it's a level playing field..tall, short, fat, thin, old or young-- everyone will be awkward the first couple of months... a good Sensei knows this and encourages his/her students to work on improving their weaknesses and harnessing their strengths. Some use kata's as a learning tool others don't.
I recommend trying to be as informed as possible on the different styles of martial arts before choosing. Every style is unique and generally the physical demands can be quite different.
For example Aikido and Karate and Jitsu are as different as day and night.
Aikido incorporates a wide range of techniques which use principles of energy and motion to redirect, neutralize and control attackers. At its highest level, aikido can be used to defend oneself without causing serious injury to either the aggressor or the defender. If performed correctly, size and strength are not important for efficiency in the techniques. Aikido is considered one of the most difficult of the Japanese martial arts to gain proficiency in.
Within karate there are presently a multitude of different styles or schools. Karate emphasizes striking techniques, such as punching and kicking, knee/elbow strikes and open hand techniques. Mostly keeps the student on the offensive. Defense is taught but at higher levels. However, grappling, joint manipulations, locks, restraints, throwing and vital point striking are inherent in the finer points of the kata. Beware:many "freestyle" schools in the West (particularly the USA) sell a highly compromised interpretation of the art, and should not be regarded as emblematic of karate. The freestyle approach is oriented heavily towards sport competition, which includes point fighting and demonstration of forms (aka, kata) for entertainment value. Martial practicality is eschewed in favor of gymnastic and musical fashion. Not very practical for self-defense.
Some define jujitsu and similar arts rather narrowly as "unarmed" close combat systems used to defeat or control an enemy who is similarly unarmed. From a broader point of view, based on the curricula of many of the classical Japanese arts themselves, however, these arts may perhaps be more accurately defined as unarmed methods of dealing with an enemy who was armed. An all-encompassing awareness, zanshin (literally "remaining spirit"), in which the practitioner is ready for anything, at any time; the spontaneity of mushin (literally "no mind") which allows immediate action without conscious thought; and a state of equanimity or imperturbability known as fudoshin (literally "immovable mind").Together, these states of mind tremendously strengthen the jujitsu practitioner, allowing him the utmost potential for effective action. Such effectiveness and the technical competence and mental mastery on which it stands, however, is possible only after a period of serious and devoted training.
Regardless of what martial art your son may choose:
Check their pedigree(they should all have one)
Find out about their roots..how long have they been teaching? Who taught them? etc...
A good Sensei will teach the basic fundamentals on how to fall properly without injury before anything else and will also instill the weight of responsability that this art demands. Every strike was designed to injure...my teacher taught me that the best fight was the one not fought.
Now I've said a mouthful... hope this helps!
GOOD for you Mike!! It is frightfully hard to peel off pounds the hard way (naturally - no stomach stapling or crazy diets) Keep up with the walking..it is easy, it is free and it really is good exercise! I had thyroid cancer in 2003 and before it was diagnosed I had gradually been gaining weight. Now after 2 surgeries and treatments they have me on meds to get my body doing what it should and supposedly my metabolism is supposed to start working again! (From their lips to God's ear) So, I do understand how hard it is every day. Which is why my doctor told me to add something new. He said..."Amy you have great legs, it shows you walk but your muscles are too used to it, you need to add something new." I've just added some free weights for my upper body and am considering biking...but I am truly not an athletic sort...so finding something else I can do is interesting.... 
The endorphin high is amazing -- they make very persistent and devoted trainers. 
I know its only been 25 minutes since I last posted but do hope you get back into it..Really..
No in all honesty, let him have a try at the martial arts.. I have seen kids as young as 5 which is really to young they don't have the attention span-but 9 is the perfect age-the boys always feel so cool taking it- they soon learn its not something you are suppose to go around bragging so that makes it even more cool..