C.,
This is what my son used to do...before I took him to my Podiatrist and asked him to check it out. He recommended a combo of Physical Therapy and Occupational Therapy, to help with the 'calcification of the ankles'...sounds more serious than it is, but without therapy it would require leg braces for 6 months on and 6 months off.
PT/OT was the best thing I ever did for my son. He was only 11 months old when we started and that was when he began to 'tip toe' walk and scoot rather than crawl/cruise/walk. I also had to make sure her wore shoes for 2 hours in the morning and evening and of course whenever we were out. This helped his feet to 'know' what to do.
I think for most kids it is supposed to work itself out, but I'm one of those Moms that gets second and third opinions on stuff before I decided it's nothing. This could be nothing, but it could be something and it never hurts to ask.
As for your step-daughter, this and any subject you broach as concern might cause issues, just because as a Mom we like to think we're doing the best we can. First, do some research! Then, I would casually just mention you noticed he's a 'tip toe' walker, has the Pedi mentioned anything at his last appointments? Make it a very casual conversation and try not to make it like you're 'worried' (while you are), but just an observation. She may not have really noticed it and your pointing it out might help her see something that she didn't before.
I know with my Mom, when she says things in a non-threatening way, it is easier than if she makes it a big deal. We live with my parents, so I've learned a lot about listening to the 'wiser' Mom who has more experience and then also taking concerns with a grain of salt. Just be loving and kind and open to helping her figure out if it's an issue. I think that's the best thing, is when someone is offering help and making life a bit easier, rather than telling you something is wrong.
Good Luck!