Things will go much more smoothly if you just give him the toothbrush. Let him hold it and chew on it and suck the toothpaste off of it (fluoride-free, of course). Sit him up on the bathroom counter, with his feet dangling in the sink, and everyone can brush their teeth as a family. Your goal right now should be about the experience and routine, not technique. We started this family brushing routine with both of my kids at age 6 months. Now, at 17 months and 3 1/2 years they have never had an issue with brushing their teeth. After a while of letting them do it themselves, you should be able to say "Ok! great job! Now it's Mommy's turn." Then you gently brush whatever teeth you can get to while making "eeee" and "ahhhhh" faces and fun sound effects. We had electronic toothbrushes when my daughter was little, so we would make "motor" sounds when she was learning to brush teeth. We have since switched back to regular brushes, but we still make the sounds for my son, and he loves it. He is only just now letting me actually get the brush in his mouth when it's my turn with his teeth. For months it's basically been me holding the brush in his closed mouth while he giggles. My daughter lets me get in there pretty good now, and she had a VERY successful first dental visit about a month ago. She was so comfortable that the hygenist actually got to do a full cleaning, which is rare for a 3-yr-old (or so she said). In fact, I use that visit to my advantage and tell her, "Now let me clean them really well like at Dr. Taylor's office."
You can also say things like "Oh, I really like how shiny your teeth look when they're brushed." If my daughter has ever been reluctant to brush, I will say something like, "But I think I saw some Cheerios stuck in there, you might want to brush it out." She usually has to at least look in the mirror for the stuck Cheerios, then we have fun making up crazy things to brush out..."Oh my! Is that an elephant on a back tooth? Do you want to brush it out, or shall I?"
The routine of it is really important. Just hand him a tooth brush every day at the same time. He's young enough that it shouldn't take long to convert him to a very happy teeth brusher.
It's so ingrained in my kids now that I don't have to say a word about it. It's funny to see the baby dashing down the hall at 7:30, heading to the bathroom babbling, "brush teeth! brush teeth!"
Honestly, I don't know a single toddler whose teeth have needed dental work or rotted out of their head. My sister's kids never saw a toothbrush til they started school...and they are practically given Dr. Pepper in their bottles...and still no problems (until they were older, of course).
Think of baby teeth as a "practice" set, relax, and have fun!