Your conversation last night confirmed that he is already seeing other options, but I want to come at this from a different angle.
When someone has an affair, it is necessary for them to detach emotionally from their partner, to stop "giving" to the relationship. Now, in a happy marriage where the connection and attachment has always been strong, usually this distancing starts only after a likely affair partner has materialised.
Because most people have been brought up on a diet of beliefs that affairs don't happen in happy marriages, when a temptation arises and a person deep down wants to say "yes", if their marriage is good, happy and fulfilling in every way, they cannot allow themselves to act on their temptation. So in order to say "yes" (which is what they really want to do) they simply have to detach from their primary partner and reduce the connection.
However, sometimes affairs happen long after somebody has detached from their partner and the only reason they haven't been unfaithful beforehand is because of the absence of opportunity with a suitable affair partner.
If he is willing to go to counselling on his own, these are good signs, because it means that he is still giving enough to your relationship to process his feelings. But you might ask him to talk with his therapist about his attachment styles to care-givers as a child and see links between this and his current attachment to you.
What I want you to consider today is your H's detachment from the relationship and crucially, when you think this started.
Some of the things he is saying to you speak volumes about how detached he is from you and how lacking in empathy he is for your feelings at the moment. He is starting to re-write history, wondering aloud whether he ever loved you properly and it was just his need to "rescue", yet he finds himself attracted to someone now, who is bereaved and vulnerable. Is there anything about this friend of yours that he mentioned, who could fall into the same category, I wonder?
You know yourself that when a partner says he loves you but is not in love with you, it very often means he is in love with someone else and I wonder whether your counsellor has directed you to this site because there are so many threads on here where affairs have been uncovered after a spouse has heard that hurtful statement?
Since your H is by his own admission, already engaging in an "auditioning" process for likely partners, if you just take him at his word and he is not being disengenuous about existing involvement, what you cannot ignore is that he is looking outside of the relationship already.
What I am saying to you is that to do that alone, a person needs to have detached from their partner somewhat and since he has now convinced himself that he is not in love with you and wonders if he ever was, that detachment is pretty much complete. There are no emotional barriers now in terms of you and your relationship, that would prevent an affair. All that's left are his own values and moral code about infidelity.
Focus on detachment from you and read around old threads today. Write down how you feel and start timelining when you think this detachment started.