The fact is his mom is the type to send her the most embarrassing pictures possible, and he knows it. He likes her and doesn’t want her to see stuff like that when he’s trying (at least subconsciously) to get her to see him as a man.
He doesn’t want to be undermined by his own mom. She already likes him, and his mom is actually an amazing wing-woman so it’s all moot of course, but he doesn’t know that.