Everybody does it. If he notices, or if anyone notices, depending on the situation be really polite and say "Excuse me." Or be humorous and say, "Whoopsie!" Nobody really believes that people can make themselves "never" fart in public.
It depends on the type of person you're dating. Girls generally are more tolerant of their boyfriends farting; whereas guys sometimes can be freaked out by girls doing it. My suggestion is to silently pass gas somewhere slightly occupied so you can blame it on a passerby if he/she gets grossed out. Then make a joke about it to gauge his/her reaction. That will give you an indication of where your significant other stands
Personally, I think farting should be left for the bathroom, no matter who you're with. But there is always the accidental "slip", if that happens just apologize and excuse yourself. It happens to everyone.
depending on ur relationship and personal tolerances...lol...my bf makes it a point to let'em fly since i made the mistake 3.5years ago to excuse his "oops", which now i regret cuz it happens so frequent he does it just to annoy me