what is the best legal path for me if I want my father to stay here in USA with me ?

my father is visiting me from bangladesh on B2 visa. he is here since 7th may 2018. his stay permission is until 7th november 2018. at this point I want him to stay longer than that and if possible also want to change his status to immigrant to avoid this visa situation in future. i am an US citizen. my dad is 72 and physical condition is not very favorable to fly back and forth from Bangladesh which is a long flight. please show me the right path to follow.