You should absolutely take your cat to the vet, as soon as you can if you suspect a broken leg. Although cats are remarkable at healing in certain situations, sometimes this can be detrimental if a bone heals in the wrong position as it can affect him for the rest of his life.
Also bear in mind that a broken leg is extremely painful, although many cats will still look quite comfortable. He should have the benefit of the pain relief that a vet can provide. Your vet will need to also check for any other injuries - if he has suffered anough trauma to possibly break his leg there may be other internal injuries that aren;t so clear.
Your vet will probably xray the leg to dtermine if and where it is broken, and determine the best way to go about a repair, whether it be a splint on the outside or internal fixation. Fractures take approximateyl 6 weeks to become really stable depending on how complicated they are. If there is any open wound to the keg then this will need to be examined and treated with antibiotocs to prevent a bone infection.
Until you can get to the vet (which i would do straight away) definitely keep the cat indoors and conifed to just one room so that he cannot damage himself further. Going out with a broken leg could result in him not being able to run away from a fight or a motor vehicle, or have another accident trying to get around.
I hope this helps,