S.H.
There are so many different martial arts.
So, with that in mind, you find one that "speaks" to your son.
Since he will be taking it.
My daughter takes Karate.
Husband is into Aikido.
My daughter also tried Tae Kwon Do.
ALL of these, are different.
My daughter, of her own volition, wanted to take a martial art. We looked around and she tried several. SHE chose, Karate. It "spoke" to her. She connected with it. LOVES it to this day. And still takes lessons.
Husband on the other hand, loves Aikido. For his own reasons.
Each Dojo (school) and Sensei (teachers) and martial art, are different.
They are not all the same.
Some are trendier. Some are more traditional.
They all teach discipline, philosophy of the art, listening, self control, respect, etc. BUT each martial art... has its own approaches and way of life, etc.
AND the prices for it, WILL VARY.
In my city for example, Taekwondo, is very expensive. For some reason. And there are MANY Tae Kwan Do "schools" here. It is trendy. And even at 7 years old, they can be a "black belt" already! Not that the kid is REALLY, of black belt ability.
Again, some schools are trendy. Some teach traditionally
They are not all the same. Even within the same martial art.
If your son likes it, and going to it is not a battle, then that's good!
*ETA: at the Dojos that my daughter and Husband attend, the Sensei are volunteers. DAILY. They do not get paid. They have done this for years. Decades. They are very high ranking in their respective martial art both locally and regionally/internationally. They do NOT do it for profit. They charge monthly, and it is not much, in comparison to other things/Dojos/Sports. And some of these Sensei, are even Police Officers. Who have actual field experience and STILL... study their art and compete and have been earned higher degree belts. Still. Even at their age. Their Dojos, have a direct connection, to their Japan Dojos and the Sensei and Founders there. They are taught, in very traditional ways.
Some of the martial art schools here, Taekwondo, teach it in a trendier "all" kids get a trophy or belt, type fashion. It is in terms of physical activity... not the "art" or mastery of it, or truly earning it... in a traditional martial arts manner. Hence, SOME of the kids, even at 7 years old, get a "black belt." Of course, they are not an actual black belt in terms of mastery and knowledge, of the marital art.
The Dojos my daughter and Husband attend, per payments at their respective martial arts lessons, do NOT lock you in, for quarterly payments or refuse to refund you if you change your mind. Participants are, allowed to pay monthly, or for several months at a time, IF they want to. The monthly tuition is not a lot. However, the Taekwondo schools, here at least, for some reason, charge a lot, and you have to pretty much pay in advance for at least 3 months at a time, plus there are other things to pay for as well here and there.