cookies!
give him some soft cookies and dip them in milk. after he does tell him to drink some milk.
or
you can tell him there is something special about it. like if he drinks it, he'll get bigger, or something he wants. maybe tell him if he drinks it he'll get a special toy under his...
Pollen isn't really an issue, it's just like flowers falling. It's a plant in which some people are allergic to also. It's nothing to serious though. It may just be where you live, nothing to get worried about.