There is acknowledging that this is something you wish to do -- fluid bond with other partner -- BF2.
There is acknowledging that right when BF1 broke up with his other GF.
So... perhaps give him a reasonable time. A week? A month? I don't know what time frame is "reasonable" in your situation over there. Maybe he's not all that upset over it? Maybe he is?
But I do know it is unseemly to bring it up the day after a break up! Give him his time out time to gather himself a bit.
You could tell him you want to make a date for serious relationship talk -- on health issues in future. Nobody is ill or sick but -- a relationship is deepening so... let's talk.
Then when appt time comes -- tell him you want to fluid bond with other partner. Where is BF1's preferences there? Does he feel willing to "overlap?" Does he want to go back to condoms with him?
While you are waiting in the meanwhile -- you and BF2 can make sure your health tests are up to date and take new ones. Not just because BF1 may want a gander at those if he's thinking he might be ok with overlap, but because it is management of your OWN health and it's the responsible, ethical thing to be doing when you share multi-partners. Tend to the health bucket.
I don't know if that helps.