without a vet..... hm...
the best you can do is as Caspian advised to set up hospital set up. are you sure it is broken? if so you need to make sure a toad is in tiny space so he can't move much and pretty much would sit in one spot. you can't really properly immobilize the leg with anything yourself. your best bet is just to try to keep him as less moving as possible, if there wounds treat with antibiotic cream, make sure it is WITHOUT painkillers, you can soak in melafix ( fish medication) diluted according to the instructions on a bottle.
watch him for any signs of anything, his internal organs might be injured due to impact.
without vet you can only hope for the best and that whatever he got damaged will heal