Full custody

Hey ladies. I’ve been going back and forth for quite some time now and I’ve decided I want to leave my husband. My issue is custody of my son. I do not trust my husband. He is very careless and unreliable and a drinker. I have no way to prove any of this in court obviously. The only thing stopping me from actually filing for divorce is this. For example , not once, not twice , but three times when my husband was with the baby he put him in his car seat , did not strap him in at all and drove away like that. You can see my concern. Again I have no concrete way of proving he is unfit. Any suggestions?