J.S.
Hi M.. I would try the soft nipple sippys so they seem like a bottle. Also, I got this from the WIC office, to dilute his milk in his bottle. Give him 4oz milk 2 oz water. Do not offer anything else in the bottle. If he wants pure milk, juice or water or whatever, only offer it in the sippy. After a day or two dilute the milk again in the bottle to half milk half water. Then to 4oz water 2 oz milk. Keep offering pure milk and juices and such in the sippy. Then you'll get to only water in the bottle and everything with flavor in the sippy. This is what I did with my daughter and after the second time diluting the bottle she wouldnt take it. She also would only use the soft nipple sippys... we tried a lot of different types till we found one she liked. Maybe take your son to the store with you to choose a sippy himself.
Well, hope this helps! GOOD LUCK!
J.