or do you just wait for a ring first?
You don't have to be married to get her something for Mother's Day. It's a nice gesture nonetheless, and if you're close to her and you act like family then you probably should get her something. Just make sure your boyfrend knows it's not some way of hinting at marriage, you might freak him out :O
I'm not sure there is a time frame. Maybe when you feel comfortable. Or when in doubt, a card is a great way to say you care with out the awkwardness of getting her the wrong thing or too early.
I don't think that you'd need to be married or engaged to get her something. I feel that it'd be nice for when she knows that you and her son are serious, or if you and his mom are close enough.
He won't the only reason we're not is my father. His family is just waiting for the ring at this point.