If he refuses to groom it off, you will need to put it down his throat. Sorry.

Put him on your lap, facing out, arm around him and open his jaw from the side of his mouth, using your finger and thumb one on each side, in the join so he can't close it. Use a syringe and put it quite far back in his mouth, under or to the side of his tongue behind the joint of his jaw. (Stick your finger in your mouth and find the spot I am talking about - where you can feel the end of your gums, the soft bit right behind it)
Empty the syringe, keep holding him and close his mouth, hold it shut for a second and stroke his throat gently downwards over where his adams apple would be.

The whole process should take about 10 seconds, he won't be thrilled, but it doesn't hurt and he will be fine. Give him a treat and a cuddle, and you are done.

GL! This is the only way we can get ours to take anything lol

The only other thing we did try was putting it on her feet, on the tufts in between the pads. She did lick it off, but only after walking it all over the house.