Could my son still get tooth decay even if I brush his teeth regularly?

Yes, he can. You assume that brushing will prevent cavities. Not true. Brushing and flossing correctly, fluoridation of water, and regular dental check-ups will help prevent cavities, but cannot guarantee a cavity free mouth.
Definitely, yes. Brush your teeth twice daily, floss between your teeth and avoid sugary and starchy foods. Even if you take excellent care of your teeth and gums at home, you still need to see your dentist regularly.
Yes. What you are doing is great (depending upon the age of the child) and will go a long way to preventing tooth decay. However there are factors that can still lead to decay. For instance, do you brush and floss soon after eating? Does he avoid sticky, gooey, sugary snacks and sodas? I once had a child who never had a cavity until her mom thought fruit roll-ups was healthy. Then she had 13 cavities.