Why not both? My son is close to both me and his step-mom. For his wedding, he did a mother/son dance with me, and then one with his step-mom. It was lovely, and I'm really glad we both got those memories. If your mom complains, ignore her or tell her to keep it to herself.
You should confront him by confronting him! Dont over complicate a simple thing my friend!
I get the impression you may not actually have met in person. If so, that would be an obvious reason for her holding back.
Why not both? My son is close to both me and his step-mom. For his wedding, he did a mother/son dance with me, and then one with his step-mom. It was lovely, and I'm really glad we both got those memories. If your mom complains, ignore her or tell her to keep it to herself.