What should we do, or how can we tell him in a way that will not result in him killing us in a serial murderer type fashion?
There is no such thing as "etiquette" in this situation. You have to choose whether your own happiness pursuing this relationship matters more than hurting the exspouse or the gossip of others.
Recently I got divorced; the hookups and sleepovers with Jeremy have become much more frequent.
Some of our friends have learned haphazardly of our relationship.
It has gotten to be a bit more serious with the added complication of a terminated pregnancy (this caused us to appear stressed in front of our common friends and triggered some of them to ask questions).
At this point, Jeremy and I are certain that most of our friends will learn of our relationship, as well as my ex-husband and we don't know how to approach the situation of letting him know without hurting him.
You started up a relationship with one of your exhusband's friends.
I was married for approximately 5 years when about a year ago I began spending some time with a new friend.
This friendship became a little complicated with very occasional hookups over the last 6 months.