S.H.
1) if using a brush is hard, use a dampened cloth and wipe all his teeth with it. That is what our Dentist suggested.
2) He can use toothpaste... try different types/flavors and brands.
3) brush twice a day. At least. And after sweet/sticky foods, have him rinse with a cup of water. If he can do that, since he is young.
"Spiffies" teeth wipes are also great. It is made with Xylitol... which prevents cavities. It comes in different flavors and can be found at CVS stores, or Amazon, or at www.spiffies.com
Even I use it myself. It is real great. My kids like it.
You can use it on the go too... and keep it in your purse for example.
To brush his teeth, so as Dentists do: have him in your lap, facing away from you, then from behind, wipe his teeth... or, have him sit somewhere where he can be distracted, or have him sit on a stool at the sink and crouch down to his height and clean his teeth that way.
good luck,
Susan