My sympathies to you and your DS. My 5yo broke her femur this summer as well. She had slightly different treatment, 3+ weeks in traction followed by 3 weeks in a spica, followed by 2 more weeks of no weight on it at all. The traction was because it was a displaced fracture, so to pull it back into position.
Like your DS once she started walking, she's gradually got stronger, although she too was walking with a limp and her foot turned out.
However, unlike your DS she has been seeing a physio to help her walking the whole time. She's still going to the odd physio appointment now. The exercises have been really useful in helping her. She's older than your DS, so maybe that's why, I'm not sure, but I think it's been really useful.
I get the impression that the hospital doctors' attitude is, once the bone is fixed, children will be okay, because they'll grow out of any problems. Whether they offer physio probably depends on the child's age and the attitude of the local health trust, but if I were you, I'd go back to your GP and ask for some physio. They might say no, but it's worth asking.
The other thing we've done, independently of the physio is take her to see an osteopath. I don't know where you are located, you get osteopaths everywhere, but if you're anywhere near London, I would really recommend the British School of Osteopathy. They have a children's clinic and appointments are cheap at only £11 a session, as it's a training school and students sit in on the session. The osteopathy has really helped in terms of realigning her spine, her muscles and everything else that has been thrown out by the limping and just the body's reaction to the fracture in general.
Hope your son is back to walking/running fluidly soon. I have banned anyone from breaking anything ever again in our house! Don't know that it will work, but fingers crossed.