Everyone may not agree with me on this, but I think, based on what you've told us, that you are probably overreacting. Although his personal assistant is a woman, it seems as though they are very good friends. Traditionally a person is "supposed" to have a best friend of the same sex, but that is not always the case. Of my wife's two best friends (she oscillates between which one holds the top spot depending on their recent interactions), one is a lesbian and one is her ex boyfriend of three years. This doesn't really bother me, though. The biggest possible threat is her ex, but they ended things on very bad terms and for very good reasons. They know that they can't be in a romantic relationship together, but my wife was with him in the hospital after he was in a near fatal car accident. She sat with him while he was in a comma and didn't sleep or eat for nearly two weeks while he was in there. That's something that keeps them tied together, whether they like it or not, and as the wounds of the relationship have had time to heal they've become good friends.
As for setting boundaries, though, I think you're in the right place, but you need to do it in a way that will not make him resent you. For example, it's good to have some boundaries in your life between work and home life, since otherwise you never get to relax! Try making that case that he will be happier if he does this, and only if you can't convince him should you ask for the boundary directly, and even then frame it in terms of emotional needs rather than a demand. "Honey, I want to enjoy the time we spend together. You taking work calls during that time detracts from it. Can you try to limit your work calls to emergencies only when we're together so we can better enjoy our relationship?" If you demand things of him he will start to resent you and this can lead to passive-aggressive behavior that is damaging for the relationship.
So, encourage him to make improvements, but consider how your thoughts may be affecting the relationship. Are you making him choose between you and a good friend? Is there any reason to believe that he is having an EA or PA? If not, why do you feel so jealous? Is there something from your FOO that is causing these feelings in you? Try to think about how your thinking would appear to your boyfriend. Then you will have a better understanding of whether or not you need to feel jealous.
