Do you think it's bad if I ask my neighbor to stop singing?
Why do you want to ask your neighbor to stop singing?
He sings really load and off-key, I can hear him through my apartment walls.
If his singing is so loud and annoying that you can hear it through your apartment walls then you should ask him to stop singing. Tell him what the issue is and try to find a compromise.
I'm not sure that he'll listen and I'm afraid that I'll have to keep listening to him.
If you speak to him and he's unwilling to stop, talk to your landlord about it. He may be violating a noise ordinance and may be required to stop or reduce the volume of his singing.