Good Morning Y. -
I have 3 children so I have been through this too. My son had the Triple Terrible Threes!! He is 9 now and still has to repeat cleaning his teeth at least 4 days out of the week. I still do daily "inspections" to check all of their teeth - yes even the 12 year old (who actually is more particular with her teeth than I am)!
You have received some fantastic advice already all I want to say is stay strong, be in charge, this is not an area that he can win in.
I am very proud to say that my children are 9, just 11 and 12+ and have no fillings or cavities!! So all that persistance paid off when they were 1,2,3,4,5. It is part of their routine - twice a day. Also they are not allowed to eat or drink anything after cleaning their teeth at night.
We tried the timed toothbrushes, the musical ones, the diclosing mouth wash - makes plaque turn blue - all kinds of toothpaste, but kids will find a way to make a game out of all of it in the end. Try singing him a song that last for 2 minutes instead and save your money honey.
Good luck
M. F