V.
Yes, it is necessary to cavities fixed on baby teeth. Those teeth are meant to last until your son is about 10-11 yrs old. If you leave a cavity untreated for 7 years, it will abcess and cause pain. Then it will have to be pulled, and the space it was holding for the permanent tooth will be lost, and the permanent teeth will be really crooked.
If the cavity is caught early enough, instead of a traditonal filling, the dentist may do a resin restoration, which is basically like a sealant. Sealants are usually done on the permanent molars, which come in around age six. usually, a restoration like a sealant is cheaper than an amalgam filling.
If the jaw is not aligned now, there is an appliance that can be made to expand the top jaw (if that's the problem)and avoid full braces later. It's usually done around 7 yrs of age, while the bone is still growing, and it could be "guided" to grow the correct way. If it's done right and on time, it could save you thousands of dollars in braces later.
If you have any other questions like this, feel free to email me with any and all of them. I work at the Infant Welfare Society dental clinic and we see everything.
By the way, February is Children's Dental Health Month, so you're very timely!