Start out slow...my dentist was very clear that the night brushing should be prioritized over the morning brushing to ensure they are all nice and clean before bed. If any of his teeth touch each other, you should also be flossing--there are some great pre-threaded flossers that are sized for a toddler mouth.
Get a couple of different toothbushes and let him choose which he would like to use. You can also try two different flavors of paste.
YOU need to be brushing his teeth, but I let my daughter start and I finish.
I ask her to show me her "monster" teeth to open wide and her "smile" teeth to let me get the front and sides. She reminds ME to do her tongue. I also encourage her to SPIT in the sink all she wants while we are brushing. She thinks this is great.
Don't stress if you miss a night here and there...just try to get him to rinse his mouth with some water at a minimum.