This is definitely a subject to talk to your son about. I think the videos are a good start. Explain to him that there are some rules that, no matter what a trusted adult says, are NOT ok to break. Explain to him that you know his Daddy says it's ok to go without and that sometimes Daddy may have different rules from Mommy, but in this case it's up to him to do the right thing and to always wear his seat belt. Don't make it into a Mom vs Dad subject. You will lose everytime and your son will quickly ignore your warnings. Stress the importance of seatbelts and why they are helpful. Help him learn about the reasons seatbelt wearing is mandatory. Talk to your ex as politely as you can and tell him that although you may parent differently and have different house rules that you really need to work as a team and make safety rules the same so you can both do your part in keeping your son safe. Make sure he understands you are not picking a fight or just being the "crazy ex"giving him a hard time but that this sincerely worries you. Find a way to communicate and ask him to please make sure your son is wearing his seat belt. If this does not work I would consider speaking to your attorney for further advice.

@lakota-winyan, I showed him some videos and he said he would but I’m hoping he actually does it so I’ll have to tell him I seen him in a friends car and I seen he wasn’t wearing it

I’d also install in the sons head that he HAS to wear it at all times, even though his dad and grandparents think it’s ok. I’ll let my son know the consequences of not wearing one also. Because it’s sad to know that no matter how many times you tell them to make him wear his belt, you never know if he is. If your son knows what can happen he may be more likely to buckle himself in whenever he is in a car.

That is so scary! I hope you figure something out. If the father didn’t listen I would call and report him after they leave. Let him get one of those big ass fines for it and see how much he does it after that.