Her primary allegiance is her son.
For years my grandmother would talk about the beautiful, talented Christian woman that my uncle dated. The woman he married was witty, loyal and a great seamstress.... but never was as good as that ex-gf.
If your bf's mother thinks he's mono... she's likely to be friendly, but avoid being too friendly. Maybe she's waiting for him to select his wife....and staying out of the selection process... not realizing that he can have more than one partner.
I think your feelings are causing you to hyperanalyze the situation. Relax, enjoy. Learn about him through his mother's eyes... what was he like as a child? What challenges did he face? etc..