My baby only bites me, too. But, I think it's because she loves me more! LOL. Anyone else can put their fingers in her mouth and she'll lick them. But if I so much as bend down and my hands are anywhere near her reach, GAME ON. I really don't mind because she'll stop when I tell her 'No biting' and, for me, that's the key. My hands just happen to be her favorite toy because I've always allowed it as long as she doesn't bite hard. No one else in the family would tolerate such foolishness so she knows better.
Just give your little one a 'No biting' or similar command and redirect him to another toy. It sounds like he's playing with his Mommy, but may need a little correction to be a bit more gentle.