What are you using to brush them with? I bought the Oragel flavored gum and teeth cleaner with the little finger brush and once my daughter tastes the flavors, she's more likely to let me do it. Although, when she has a tooth coming in, she doesn't let me do it, so I just don't. ...and I get bit a lot!
Others may disagree with me, but I think although it's important to keep their teeth clean, at this age you don't want to freak them out about cleaning their teeth, so I'd say if you have to hold him down, then let it go until he's ready.
Good luck...and watch out for those finger bites!