Sorry to say your dad is mistaken- heat rises, & UTH (heat mat) NEEDS to be under the bottom so the heat rises into where the snake can be over it. If the UTH is incorrectly installed on the side, most of the heat will go straight UP & away, & do nothing for the snake, unless of course you can teach him to stand up next to it or somehow stick to the side like a gecko.(& you can't)
You will need to also check the temps. very carefully once the UTH is properly installed- it's not about what you set the thermostat to- you need to check what the temp. is inside where the snake can make contact, because he'll either be safe, healthy & warm if it's correct (88-90* maximum), or burned (injured) & needing the vet if it's too hot. A few degrees matters greatly.