Is he mobile yet? I know Tyler went through a phase right before he started crawling and every so often after that where he would fight being restrained...sometimes the high chair, sometimes the car seat, sometimes the stroller. If he calms down after you start feeding him, your best bet is to ignore it for now and wait for it to pass. If you aren't already, it might help to have his food all ready to go so you'll have something to distract him quickly (sometimes being silly or singing a song can work too). Do your best not to react to it at all, it will pass!