My daughter is now five. At 2 1/2 I made her give up her bottle. I had a purchased every kind of sippy cup made trying to find one that she would drink milk out of; she wasn't interested in any of them. One day I couldn't stand it anymore, and very loudly threw out all her sippy cups, which got her very upset. But I realized a couple of things after this incident.
She likes her milk warm and she liked the cuddle and comfort of drinking milk from her bottle. I decided the warm milk wasn't an issue, so I continue to warm her milk. She prefers soft straws on covered cups rather than hard plastic sippy cups. When she first switched to a straw cup, she sat in my lap to drink her milk, so she still got the cuddle and comfort. I still bring her milk in the morning, so I know she's gotten some hydration right awasy (hey, my husband brings me coffee; it's just a nice thing to do) and she drinks milk for breakfast, so she is getting the recommended amount of milk first thing. When my mother got me off the bottle (finally), I refused to drink milk at all, much to her chagrin. I add a splash of vanilla Pediasure to my daughter's milk, just to be sure she still likes it. Low-fat chocolate milk is okay, too. It has less sugar than fruit drinks.
How long will your son be in preschool? If he is only going to be in school for three hours, just pack a light snack with a box drink. You can buy milk in small boxes, too. BJ's sells New Horizon organic milk drink boxes for a reasonable price. But don't send a bottle, the teacher is not going to mess with it at all, and you might get an earful from her about "babying" your son.
I hope your son is out of the high chair/booster seat by now, so potential mess and spills can be an issue. With a one year old and a baby on the way you don't need to make extra work for yourself. I don't think the straw covered cups are as questionable as the spouted sippy cups. Adults still drink through straws, and since the drink is directed more to the back of your mouth, past the teeth, the liquid is less likely to wash against your teeth.
By the way, you can't see cavaties. The dentist finds them with a probe because they start out as soft spots in the enamel. So I echo the other posters that you need to get your son to the dentist asap. Some dentists think you should bring in your child when they get their first tooth. Others say wait until the child is 2 1/2 or three. If you see a rotten tooth, it's too late and the tooth is lost. And improper dental hygiene is not always the issue. My daughter had three cavaties in her molars because the groves in her molars are so deep. No amount of brushing would have prevented the cavaties. You can also buy pre-threaded flossers with a small arm spread (I think it's make by Oral-B). You can teach your son how to floss without having to wrap floss around your fingers or his.
I hope this helps. Your son probably still lides the bottle because of the comfort associated with it. You can get rid of the bottle but still keep the comfort.