It depends on what kind of martial art your son is going to practice, but yes, a mouthbuard would be the first thing I'd recommend for any kind of contact sport (so even i.e. for basket ball). He should wear it any time he does an exercise with a partner and not only when he's fighting. Any other part of the body can heal, but a broken tooth will not. It will also help him for his fighting performance, as there are many people who wear their mouthguard only when fighting and then have problems at breathing with it, because they are not used to wearing it.
Next gear would be a groin cup. Any other type of protective gear depends on the type of martial art he's going to do. His trainer should be able to give him/you specific advise.