You can start brushing as soon as the teeth come through - either using your finger or a small toothbrush (most supermarkets sell age 0-2 ones, or you can get a rubber one that goes over your finger. I use Milk Teeth toothpaste - just a tiny bit, so there isn't much to swallow.
I've bushed both of mine's teeth as soon as they came through, ds at 4 months and dd at nearly 7 months - I did wonder whether to start dd's earlier, once she'd started weaning, it didn't seem right giving her food and not brushing her teeth, even though she didn't have any - I got mixed responses, there was no need to brush her gums but also no harm in going through the motions to get her used to it.
Squirtuk I'd probably brush your lo's teeth from time to time to get him used to having it done - as he's that little bit older, by the time he has teeth he might not like the idea of having it done, but no harm in getting him used to it/into the habit now - just my personal opinion though.