Having a struggling mom moment and need some other moms opinions

I have two boys one 15 and the other 11. My 15 year old is the one I’m struggling with, their father and I are separated and have been for 9 years. I purchased my son a cell phone last year, as he’s been bugging for one and I felt it was time. Months later when he goes to his dads he lets it go dead for days, doesn’t touch it for days even… I asked what the issue is he shrugs his shoulders and tried to tell me when he’s over there he wants to spend time with dad. I honestly feel like this is an excuse. Their father is not someone you can talk to, so I cant ask him. He’s never on the same page as I am with the kids regardless of the situation. My son has said the cell phone is not an issue with his dad. I’ve explained that a cell phone is a privilege and it’s also for safety (school shootings, incase someone is late picking him up, feeling sick, forgetting homework.. etc.)

I’ve taken the cell phone away before and he didn’t seem to really care, then he wanted it back cause he said he needed it to contact his friends and incase of an emergency. I have my son signed up for drivers training next month, I’m honestly contemplating canceling it… if he can’t keep a cell phone charged like he’s asked then how am I going to allow him to drive?? Please help… I’m at a loss and really struggling.