My son previously also dislike brushing teeth, but the condition is much improved after change to electronic toothbrush. You can consider to try. Of course, gum message using the special tools is also very important to decrease their sensitivity.
How old is your son? He might have sensitive gums which made him feeling uncomfortable, if he's small, you could start using those small toothbrush (without "feathers") and move around in his gums first, insist for 1-2 times a day. You could also train him for biting biscuits as well. If all these are OK, he should be OK with the toothbrush, just that you start with mild brushing first.