Thick rubber gloves or garden gloves, just cup your hands round him to hold him in one place and get someone else to spray down through the gap in your hands. Or cut the bottom out of a plastic bowl or ice cream tub and do the same although you will have to be careful not to spray his eyes. I think actually you would be better to get him by the scruff of the neck with your finger and thumb and spray quickly so it is over and done with. OK he will be upset, and probably try to bite you but if he needs spraying then he will have to put up with it.