I've struggled with DS (3.5) and teeth brushing until recently. I make sure I give him TONNES of water after every meal, limit his juice intake ( to virtually zero.... it's a rare occasion he gets juice), and make him have water before bed everynight.
One thing I found to encourage the kids to use their tooth brushes was to let them brush their teeth in the bath. They seem to enjoy having their own freedom to do as they wish.
Honestly, as long as your little man isn't sleeping with a bottle, having tonnes of sugary drinks and foods and drinks plenty of water, a short sharp brush twice a day should do it.
It's all I've done with my 2 and DS had his first dentist visit today to which the dentist commented on what wonderful healthy teeth he had. I was worried I was going to get totally flamed as it can be such a struggle at times.
DD LOVES to brush her teeth and would do it 100 times a day if I let her (sometimes I do... not with toothpaste though).
I don't like the idea of holding them down as I feel it's a negative reinforcement and will prevent children from wanting to brush.

Good luck!!!