Hm, I see what you mean.
In my personal opinion you should consider amputating that leg, I'm not sure how axolotls are with fixing their legs but I know for a fact that regeneration is not a problem.
He is probably in pain as well.
Is it possible for you to hold his live food for him? I used to do that when my axolotls were sick.
Also, I know this is not related to the broken leg, but I think your pebbles are dangerous for your axolotl.
He is looking a little plump around the tummy, which might mean he has swallowed some of those stones..
Sometimes axolotls will live peacefully with stones like that for a long time but sooner or later they will ingest one and it will cause serious problems.
For now, I reccommend you take out those little stones and just have him on barebottom tank.
The best option for substrate is children's play sand
Just make sure you wash it thoroughly before adding it to the tank, and it can take a few hours to settle depending on how much you washed it.
I hope this helped