I have a 14 year old son, who keeps insisting on joining martial arts, but I believe that he is too old to join martial arts at this age and that it will be harder for him to learn from this point forward so my question is, what martial art is suggested that he should join at this age?
There is no reason to choose one over the other based on what you have listed in your question. Martial arts that require a high level of flexibility (such as Capoeira) might be tricky if your son doesn't currently have high flexibility.
In reality, most martial arts focus on control and discipline, at least until higher levels, and can cope with entry at any age from very young to senior citizens.
I, myself, practiced Judo, Shotokan Karate, Shukokai Karate and Kuk Sool Won, and my kids have practiced Aikido and Tae Kwon Do - and the simple fact is none of them are more or less strenuous. That will depend on the class. All are approachable at any age (although I find Shotokan not so good for me now, as the deep stance causes my knees to ache)
My opinion - let him join one, but check out the instructor and class first to see how it is run. Some classes are more welcoming than others; some instructors cope better with adults, and some with kids etc;
It will give him discipline, core strength, flexibility, and even a level of self confidence.
Based on personal experience (my self and my two sons), I suggest Judo over Karate and other martial arts. Both are good for self-defense. Judo is a better workout. Judo is less likely to maim or kill a friend. Judo has more of a cognitive component as there are many different moves, especially in mat work. The people who go in for Judo tend to be nicer than than the people who go in for Karate. When your son gets older, he might want to study a more lethal martial art for even better self-defense. Your son is not too old to start learning a martial art. Many people start later than age 14.