Hi, My Dog Is An Australian Working Kelpy, He Is

Hi, my dog is an australian working kelpy, he is 18 years about. He sometimes gets shaky back legs but recently his back legts have been locking straite, he can get up but its like they go dead state and dont work if i rub them bend them then get him to walk his legs fix, the last day his front legs have been locking strate as well.