Since the beginning we have always sung a toothbrushing song with Aidyn.
DP also pretends that Aidyns brush is an electric one and makes a funny buzzing noise, and vibrates it in Aidyns mouth, and he thinks its just hilarious!
In the past I used to get DP to brush his teeth at the same time as I brushed Aidyns... that always worked well.
A new toothbrush might work too... Aidyn was reeally happy when he got his new one (a blue Hi 5 one which is probably the same as Gabbys now! LOL)

I also use a natural no-fluoride toothpaste for Aidyn... as I am not convinced that fluoride is safe. I will upgrade him onto the Milk teeth one when he can spit & rinse properly LOL