Set Default Text Message to Cell

Hello

i have a relative that has a non-apple mobile phone. However he does have a iPad that he has his personal email setup on. Whenever I want to text him, my phone automatically defaults to his email address which is registered to receive iMessages on his iPad. This can be frustrating since he will only see my text message if he looks on his iPad. I’ve tried to set it to send the text message to his cell number but it keeps defaulting back to iMessage. Does anyone know how to fix this? Thanks.