Hi & welcome:)
You seem to be a very caring and patient person and I am sorry that you are going through this. It must be very difficult.
It does seem strange that he ex should want a reconciliation after a number of years and this must be confusing for your man friend.
He has made his own decision and chosen his own path. However, I think that counselling might help him with this.
One thing, though ~ have you considered that he might be upset because you may appear to be unconcerned that your relationship with him may be over? Maybe he feels that you should not be quite so supportive of him trying for a reconciliation with another woman?? Maybe he hoped that you would tell him that she is his past and that you are his future; not just his friend??
What do you think?
"The secret of success is constancy to purpose" - Benjamin Disraeli.