The BEST answer is to get rid of all the bottles in the house TODAY so they are not a temptation or fall back for you or your husband in tough moments. Your son can clearly drink from a sippy cup (so it's not a physical issue) since you said he drinks other liquids from the sippy cups. You may want to try adding a touch of chocolate or strawberry milk - or if he is underweight, try making him protein smoothies with whey or soy protein mix (and add fruits he likes) this is what we did with my son who was also underweight and didn't like meat. Maybe he's ready for a big boy cup?
You could also let him pick out a new sippy cup and call it his "milk cup" - let him decorate it with stickers, etc. But....toss the bottles girl ;) With a new baby on the way - who may also be using bottles, your son will only cling to the notion MORE. So, having him completely weaned off of them before the new one arrives, will save your sanity!
-C.