Cathy it is wonderful that you are his best friend. I hope for both of you that he can find a way to let go of the past, you say he is mourning the betrayal and waste of time, but he may also be mourning all the hope, possibilities and love he put in to that relationship. Lets be honest after what has just happened to him, he probably cannot see a future for himself never mind with someone.
I would also say MAYBE at this moment in time he is so afraid of hurting you and losing you completely, a friendship is all he can offer, At least he gets to see you being happy.
So all I can say is be a friend, we all needs friends, just do not plan a future until he can see the possibility of a future.
As for your future and dating other people a good friend once said to me “enjoy the tapestry of life”, go dating enjoy the meeting people you always learn something new.
Love and light