I believe you can't find him anymore after battling Heatran.
go to the survival area and find Buck talk to him and he will go back to Stark Mountain with the rock. next time you go there Heatran will be hwere you and Buck found the rock. Heatran is at level 50 so bring some ultra ball's and save before you battle Heatran. Good Luck:)
Battle Tower.
to get a heatran, you have to of been to the battle grounds (bucks grandpa owns it) in the survival area then you go to the strak mountain and then you travel through it with buck until you get to heatrans cavern, then you find him and will have to battle him.
In Stark Mountain, after you go through with Buck, Heatran will be waiting.
to find heatran you team up with buck on stark moutain find buck the exit go out of stark moutain go back in go to the top of stark moutain there you find heatran
Battle Tower.
in the Pokemon diamond and pearl versions i know that the heatran (mostly a girl) will be at stark mountain go up there with buck go out talk to buck and his grampa go back up to stark muntain heatran is there
Go through Stark Mountain with Buck. After you get out, go back in and go to the place where you met Charon again. Heatran will be there.
You can find her in the Battle Tower multi battle room
You have to go in the cavern and see Buck remove the stone.
you visit buck in survival area talk to buck and go to the cave. Heatran will be there.
What do you do after you battle heatran?