Ear Defenders over bike helmet

After advice - dont know if these things exist or not. My son need ear defenders for when we go out walking or on his bike. Issues we are having is that he obviously needs to wear a helmet and ear defenders dont go over or under the helmet and also he finds the ear defenders to tight and uncomfortable after 10-15mins. Currently he is just stopping and holding is hands over his ears but this is quite dangerous when we are near roads as he distracted, loses balance or stops to hold is ears in the middle of walk ways or crossing the road, basically anywhere. What does everyone else do? 

thanks