Would my son still get tooth decay even if i brush his teeth regularly?

Yes: Brushing the teeth helps to maintain oral health. However it can be done improperly, too short of time, done often enough thru the day, etc. Flossing is essential to clean surfaces between teeth and maintain gums. Several other factors influence dental caries like what is eaten or drunk, long time exposure of teeth to acids and sugars, other health issues, etc.

Yes: Yes. Floss also. Minimum sticky foods which attach to teeth. See DDS whenever dental pain noted.