nearly 2 year old ds has a cavity in a tooth - what can I do to look after his teeth better?!

[fairylights]

we went to the dentist this morning and the dentist says that ds (nearly 2) has a small cavity in a top tooth - i was horrified tbh, i don't really give him sweets (his grandma does but she lives nearly 200 miles away!) so I am wondering how this has happened.
Is it normal to have a cavity so young?
He has a couple of snacks during the day but usually they are fruit/organix biscuits/maybe a piece of homemade cake - are there other things I could give him that would be better? (yes think we will ditch the cake..)
We brush his teeth morning and evening but maybe we aren't brushing for long enough?
I am thinking the main culprit might be drinking milk after he has brushed his teeth in the evenings?? Will def cut that out!
Any ideas/reassurance welcome!

Sometimes I feel that it is the luck of the draw! There are no doubts that if we don't look after our teeth we will suffer, however, it doesn't mean that if we scrub them constantly we won't get cavities. My twin sister has nearly every tooth in her head filled but has really looked after her teeth. I on the other hand, who has not been as good as her with mine only have four fillings at the ripe old age of 43. my DS has milk now and again after brushing teeth and I feel very guilty sometimes. I think you should be concerned about how much fruit you are giving them, as this can be very bad for teeth, the acid erodes them. You can get brushes with timers on now that go off after two minutes, they sound like a good idea I might get one for my DS as he sometimes seems to only brush them for about 30 seconds!!