Could my son still get tooth decay even if i brush his teeth regulary?

Depends.: How old is your son ? How many teeth does he have. Especially important -- what is his diet and how much and how frequently are his teeth exposed to sugar. This includes cereal, milk, soda , Gatorade, as well as sugary snacks, cakes, candy, and more. If this is high, then he is at great risk. See your/his Dentist for more thorough advice. Good for you to brush his teeth !!

It is possible: Brushing teeth may not prevent cavities but you can certainly minimize the number of cavities you get.