I had the same challenge with my LB, when the GP was giving him Rota vaccine she told us to blow on his face. That was a life saver. So just put it slowly but by bit into the cheek and if you see him ready to spit just blow on his face(short and quick) he will then gasp for air and will swallow whatever is in his mouth
I put mine in a tiny bit of milk or formula when he was little and he doesn't notice. If yours is BF could you express some and sneak it in in a bottle?
Put it into their cheek a little bit at a time and blow sharply in there face to trigger a swallow reflex. Worked like a dream for us.
Pop it in and shove him on the boob x
Into the cheek with a syringe