Wow, I am going to be more blunt.
He is either gay and hiding it from his mother with you as his beard, or that is NOT his mother and he already has a wife that he lives with.
She didn't go into details of how often they date, if he sleeps over, if she has met and interacted with the mother, or if he has a close male friend who lives in the house he bought perhaps. 
Yep there are women who can delude themselves into a decade long relationship with an unavailable man of all kinds, married, gay, or just plain uninterested.  Uninterested is the most benign of those choices which is the one Dave Eddie chose; but it usually something else, why uninterested? 
I place bets on his mother is actually his wife; he never sleeps over; she has never met the mother; she can only call him at work, so as not to disturb his mother of course, and the entire engagement is something she just wants to believe. Seen it!
He could be in the closet, but that's a smaller percentage of the population.