I would have given up my seat/space for you, but having been in your position I probably do notice more than others. My DSis is in a wheelchair now, but as a child was in the larger buggies for some time. It did get to a point where it was so much bigger than a normal buggy, and she behaved like a 6 month old, even though she was big, so it was really obvious, but unfortunately some people just see a large child in a buggy and assume you're just a lazy parent who can't be bothered to make their children walk!
My DM wouldn't have had any qualms about asking people to move (she always has been a bit outspoken!), so do try and get a few token phrases to hand (if you don't ask for assistance):
"Excuse me, I need to use the wheelchair space please"
You may come across some arseholes that will demand some sort of medical proof before giving up their precious space, so come up with a phrase that will embarrass them and make them move:
Loudly: "You really want to know the details of the disability that has left my child in a wheelchair??"
Don't call it a pushchair
"At what age does a pushchair become a wheelchair, because he's going to be in this for life."
I was on the tube once when a man got on with his toddler in a pushchair. He put the chair in the space it was supposed to go and it was facing some seats where people were sat. The little boy made eye contact with the man in front of him, pointed and said "Daddy seat" 😂 The Dad was mortified and was trying to say "no, honestly, it's fine", but the guy just apologised to the kid, laughed, stood up and let dad sit down, much to the amusement of everyone else around them!