Am a dental nurse, so will try & help.
Milk does have natural sugar in, can effect the teeth, but not as rapidly as sugar filled drinks.
As long as you brush his teeth before he goes to sleep (and from that point on has nothing except water till morning) and brush after his breakfast/morning milk then you are helping to prevent problems. Use a toothpaste with fluoride and a small brush. make sure you do it tho, as he isn't old enough to have the manual dexterity needed to brush efficiently. He can have a go after you have done it.
Try (easier said than done, I know) to get your ds off bottles an onto beakers (some of the anti-spill can cause problems as they need to suck so hard) I got for tommi tippee range, easy to use! Liquid from a bottle, once teeth are present can be detrimental to the teeth. Start taking him to the dentist on a regular basis for check - ups.
As Suzy said, watch out for sugar free products as the sweetners are questionable. I give DD pure juice, very watered down or normal squash (rare) very watered down.
Watch out for hidden sugars, they can be anywhere. Sucrose, dextrose, glucose, fructose are all sugars and can be found in many foods you wouldn't think off. (tom ketchup, crisps etc etc). Keep sugary foods/drinks to meal times, to restrict the time that suagry food spend in the mouth. Offer chocolate instead of sweets (agagin at meal time) and always offer bars, instead of tubes/bags as they tend to last longer.