Hope the poor lad is ok. Burdock is a nightmare and the little hooks on those seeds have probably got stuck in his throat
.
I purposely avoid walking anywhere that I know has burdock as they've always caused such a problem in the past just for the dogs coats resulting in me having to clip two dogs completely as it is near on impossible to remove it once it has got hold. Worse now as the seed heads are dead. There have been times when Roly has tried to remove seeds from his own coat and swallowed them, causing very similar symptoms, coughing and gagging etc. I know that I fed him little bits of fresh bread in the hope that it would dislodge the seeds would stick to it.
Big hugs to the little brown boy and hope that he is none the worse for it.