I would think it's worth buying a couple of straw cups & seeing what happens, we had one which DD didn't like very much and she much prefers a straightforward sippy with the valve removed. We also have a doidy cup which she used herself from 6 months onwards. If your DS likes an adult glass, perhaps try a doidy with a very small bit of water and/or milk and let him play with it. My DD got very wet trying at first but she has become quite proficient at drinking unaided now and I tend to keep her sippy cups for outside the home.
Have you tried many finger foods, if your DS is fussy about solids, maybe he'd be better feeding himself. Do you offer much in the way of dairy products?
I've always been much of the view that if DD doesn't want something I won't push it, do you think it's possible he's picking up on your worry about it. I know it is easier said than done, but perhaps if you try not to worry about it for a few days and let him have what he wants & no more then perhaps after a few days it will become less of a battle again.
But if after that, if you are worried, you should speak to your health visitor or similar, if only to put your mind at rest.
