My DD did that at about 12 months - went from being a great eater to flatly refusing a lot of stuff. I figured it was just her realizing that this was something that she could control and testing the boundaries. I just didn't make a fuss, kept offering whatever we were eating and I think it only took a few weeks before she was back eating me out of house and home!! We still have rough patches where all of a sudden she'll decide she won't sit on her chair but will eat when on my lap or on a big chair... or the latest is that she's decided that she doesn't like stuff all mixed together so stir-fry, soup etc is out. Siiiigh!! Hopefully it's just a phase for your DS too. I'd say as long as you're satisfied that he's getting sufficient nutrition from the formula, try not to stress too much. I know, easier said than done!