J.M.
I know it isn't easy, but if you lay your child down in between your legs-you can tuck their arms under your legs if they aren't helping. It is way better to brush their teeth than it is to have cavities taken care of when they are so little. I have a 19 month old and a 4 yr old and we toothbrush this way twice a day. I am a SAHM now but for the past 12 years I have been a pediatric dental assistant. It is really important you can see what you are doing so you can do a good job. One thing that is typically missed with kids that age is behind the front teeth and the gumline above the front teeth. Any teeth that are touching should be flossed also. We use little flossers made by dentek-you can find them in the dental section at Target or Wal-mart.
You can also let him take his turn with the flosser when you are done-same thing with the toothbrush, but it is better for you to have a different one than he uses for his turn b/c they chew them up in no time. Good luck! I know its not easy, but it sure beats having tooth trouble!