Flavored lube does help but it shouldn't be needed all that much unless the guy in question has bad hygiene (in which case get him to take a good long soapy bath before hand) or just doesn't have a palatable natural flavor.

I've got a normal sized mouth but I have had past lovers complain of too much teeth and depending on his sensitivity can be easy to get off (thirty seconds to a minute and no sore jaw) to very difficult (takes more than three minutes and you have to alternate between giving him a hand job and oral).

Depending on what he eats it can be slightly fruity to down right bitter.

However nothing matters more than opening up a dialog with this guy or others in the future about your inexperience and fears. If the guy is worth anything he will take it slow and allow you to get comfortable with the idea of oral and not get frustrated too much if it isn't what he expected it too be. Worse come to worse you don't have to do that one thing. There are so many other things you can do to please a guy.