I suggest you do nothing at all today. Maybe nothing until you've talked it over with your IC?
I think if you confront him, he's unlikely to tell you the truth anyway. He's so evasive and defensive, isn't he?
It is possible - perhaps even likely - that he's been unfaithful at some point or is being now. But until or unless he's willing to talk to you about that freely, take responsibility and come closer to you, it doesn't change anything about what is going on with you and him today. He isn't willing to do any of those things whether or not there's someone in the picture.
The other phone thing sounds extremely suspicious. I don't believe the Whatsapp excuse for one second. Unless he's in a profession where he needs a dedicated work phone he strictly isn't allowed to use for personal reasons, it fails the sniff test as an excuse for me. If that was true, he's have needed it before BD too. You not being able to go to his flat is also suspicious. Unless you've been violent towards him in the past and he has excellent reason to want to have that boundary and haven. then keeping it secret after all this time while still wanting to sleep in the family home now and again.
I don't think you're going to get the truth out of him, even if the truth is something other than infidelity. But I think you can use this information - he is closed and very possibly deceitful and keeps his life secret and isn't interested in offering you access to his heart - and use it to recast your own boundaries.
But maybe not today. Today is just for caring for yourself.
