Grandpa has an icd+pacemaker. Is it safe to wear big headphones to listen to music? I am concerned about the magnets in the headphones... Thank you!

Ok. Headphones shouldn't be a problem. Arc welding is a problem.
Probably. Who will be wearing the headphones, you or your grandfather? If you are the wearer, no worries; the magnets are too small and too far away to have any effect on his pacer. If he is wearing them, he is still probably okay - but if you're really worried, have him call his cardiologist and make sure. The big danger from headphones is turning them up too high and damaging your hearing.