My child lacks discipline, I have been told martial arts would be the best for him.

Depends if the martial arts center emphasizes it.  We allow kids to let off steam before class, but once it starts, everyone is expected to comply with the code of conduct.  There are martial arts schools that are no more than Day Care.   Fine, if that is what you want.  If you want your child to fully benefit from the martial arts, find a proper martial arts school!  Whatever age your child is, was how long it took to form his current level of discipline.  Don't expect it to change in 1 month.  Remember also, your child may start to resist going to class when and because he/she has to comply with rules of behavior.  If you see that the Instructor is actually doing a good job, do not let your child manipulate you.  When you know what is good for your child, you do not "negotiate", you make the decision!