You can't "get him to allow you to brush his teeth."
You can encourage him to brush his teeth well. Maybe you can make a game of it, and he brushes, then you do, he does, then you do, on each side, and the front. So you put a wee bit of toothpase on the brush, and he chews it off using the right side of his mouth. THen you say, "MY TURN!" (happily, please, so it's a game) and you brush those teeth without the toothpaste. The brushing is more important than the toothpaste anyway. . . Do the same on the left side, and then on the front. . . By giving him some autonomy, he feels he's growing up, and by making it a game, you are still helping, so he's getting a better job done than if he did it himself, but he's also learning the right way to do it, because while he plays (this is normal), you are actually brushing his teeth.
If that doesn't work, you can always play, "you brush mine, I'll brush yours" and let him help brush your teeth, too.
Dental Hygeine is important, don't get me wrong -- but do keep in mind that these teeth will begin to fall out in 2.5 years . . . so it isn't critical that they be brushed perfectly every time. (dentists will clobber me, I'm sure) What's important is that they learn to take care of their teeth, so when the permanent ones come in, they take good care of them. What you want to do is make it a habit, make it fun, and over time, teach him to do it well .. . he's not going to start out brushing perfectly, but over time, he'll catch on. Encourage him, and praise him when he does well . . .