It reminds me a little of the kid that lived with very strict parents going to college for the first time. There is a lot of partying and lots of "fuck-ups." Eventually they adapt or majorly screw up.
There are many ways you can respond to all of this. Let him know you feel a little ill at the thought. There is nothing wrong with that. It can take some adjusting to look at the relationship in a new way.
Ask him if he is having fun and be happy for him if he is. Yeah, it sucks that you have to use safe sex practices (hopefully he will get screened to see if he has anything and if not, you can talk about how it can change).
Try to find fun things to do. Go out on a date and have fun as well. His progress and yours do not have to be linked. He may be measuring "success" as getting laid while you may measure it as getting emotionally attached to someone.
What you have to watch out for is if he is trying to get back at you through a passive aggressive approach of sleeping with people you may disagree with. It probably wouldn't hurt to find a poly positive marriage counselor to help you two get to a good place with each other.