just wondering if it is possible, so when bowser jr dies , the level doesnt end but he just falls off and dies like on the final boss? and the level continues?

As far as I know, it isn't. Bosses are special sprites that are coded to do that. We'd have to change the game's code to modify these things

too bad. i would have thought there was something u could do so when he dies he dies like the last boss level and the level continues.

I think there is a way. You know that "bump from below platform"?
I think you can kill Bowser Jr. with it and he flies off the screen AND the level continues.
But you wont be able to kill him with lava I am afraid.