Probably an ex since Shougo's mother died. But I agree, that guy is giving off "motherly" vibes.
Though they never said that the dead wife was his mother...
Shougo did say that his parents married out of convenience.
Though it would make an interesting twist to his family if Shougo found out that his father was forced to breakup with the guy. It doesn't necessarily have to be Shougo's birth-mother, it could be his older sisters.
Since both of them look like their dad, all the options are on the table. Since the wife/mom is dead they could get that sourpuss together with the kind dude.
that the new guy is the mother-in--law or old ex of the father-in-law?