Short answer is no.
This is a common thought process that most BS ponder.
The pain, unfairness (It is much deeper than that, but that is the best label I can come up with now) and the feelings and "missing out," are common. I have had them myself. My W even suggested to give me a pass. I didn't take her up on it and told her that I would need to D first. That stopped the offer.
Your BSO is working through some very bad thoughts. It is important on how this gets approached by you. While drawing boundaries for yourself is a good thing, the fact that he told you about it leads me to believe that he is acting from a place of deep pain. He has not come to terms with the unfairness of this. It is in part trying to get you to empathize with him. He believes making you a BSO will help you understand.
I got some really good advice from another BH when I was working through the unfairness parts of the new M.
It did not go away overnight, but it helped me understand that what I really needed was to restore the balance or equity to the M.
You have to find a way to reintroduce that equity into the relationship. From his perspective he feels it is "unfair," that there were different rules for each of you. Had he known these rule at the time, he may have taken advantage. Hindsight is always 20/20.
As he has got this far and not acted on these impulses tells me it is not something he really wants to do. Desperation is a very dangerous place.
I would clearly explain that this is not something you would be comfortable with, but you can't stop him. If he does do this you would have determine at that time if you wanted to try and reconcile. Provided he doesn't tune you out I would discuss what can you do, outside of a free pass, to help him feel that some sense of equality is restored to the relationship.
Share your pain, remorse and how hard it is for you to change what made going outside this relationship a choice that was acceptable to you. Approach it with gentle humility. If he is not receptive, you may have to wait for a moment when the anger has been processed.
At the end of the day he needs to be able to be comfortable with who he is. Becoming somebody he is not shows that he let this process take something very important away from him. While he thinks it will "fix" things, based on feedback from some MH I have read, it doesn't even come close.
IC (if he isn't already) can help him. Detaching from your relationship might be a good thing for him to do as well. Letting go of the outcome is an important step and detaching (with a gifted professional) can lead him there.
PS- If he is receptive to hearing you, write him a letter he can read when he is the right mind. He can refer back to it when he gets these thoughts. My W did for me and it still helps me to read it.
He is looking to restore balance to the relationship. Explore ways outside of the current proposal that can give him that feeling until he is in a healthier and more accepting place. Show him your remorse, pain and how you have to live with yourself everyday knowing what pain you caused in him. Words are harder for him to believe, letters, actions may be the only thing he will respond to at this time. You know him better than I do, so tailor it to him.
Good luck. Sorry for the length, just trying to pay things forward for you and your BF.