Some babies decide when they are ready to take the next step in independence and this sounds like he is one of them. My oldest, girl, decided at seven weeks she didn't want the bottle anymore. She had a very strong personality even at that age. It was a power struggle for almost a week, but she then went back on the bottle, but at 8 mos, she decided again that was it. She went straight to a cup, no straw, no sippi cup; of course, I had to help her. Some children are just like that.
However, the child MUST have fluids. If you cannot get him to drink through any other means, try just a straw...no cup. Take a straw, dip into liquid, place your finger over the top of the straw to hold suction and see if he will suck the liquid out of the straw, he may also just want a plain 'big kid' cup