K., as you have already read in other responses to your question, some people are simply more prone to cavities and others seem to be naturally resistant. There is not much you can do about that particular problem. You CAN do a great deal about teaching your son the importance of good dental hygiene and instilling in him an awareness that he will always be more prone to cavities than his friends so he will need to be more conscientious about dental health.
You don't say whether the teeth with cavities are baby teeth or permanent teeth. And there is no mention as to whether you discovered these cavities while caring for your son's teeth or if you took him to a pedodontist and someone else discovered them. If a dentist reported them to you, did he or she find them by an basic examination or through X-rays? In either case, you should have him or her give you a 'guided tour' of your son's mouth and point out the cavities one by one as well as any other potential problems.
If you found them yourself, then it is high time to get your son to a dentist - a pedodontist, one who specializes in dentistry for children, is probably the best.
If the tooth is a baby tooth, you probably don't need to worry about it at all unless it is an extremely large or pernicious decay, since he will be losing his baby teeth shortly anyway. Just let it serve as a heads up for his permanent teeth and make sure he is extra careful about brushing and oral hygiene when the new teeth do come in. If, on the other hand, this is one of his first (so-called 6-year) molars, he needs to see that dentist ASAP to forestall any further damage to this or any neighboring tooth. A warning though. My son was three years old when he broke a tooth and had to have it removed. The dentist did not place a spacer in his mouth and his teeth grew in crooked. So, when you do take him to the dentist, either now or later, do your homework. Read and research all you can. Don't be afraid to ask questions about anything the dentist (or any doctor) tells you. Remember: the doctor is there to help you. You do not need to be at the mercy of any healthcare practitioner. You, ultimately, are the number one party responsible for your child's care, any medical, educational, etc. personnel are merely your support staff. You and your spouse should work together with them to make informed decisions about your son's healthcare and to provide the best care for your child.
good luck