What Can I Do Legally To Protect My Son From His Imbecile Father?

My son’s father is a fucking moron. My son is 5 and recently started seeing his dad on weekends after a year of no contact. Since he started going to his dad’s my son has explained to me that:

1. His dad locks himself in his room for his 8 hours of parenting time, and only comes out when it’s time to leave.

2. He’s around 3 other kids, between the ages of 1-6, unsupervised for 8 hours.

3. His dad recently took him to the hospital to visit his girlfriend’s kid who was admitted & under isolation precautions, now my son is running a fever & complaining of other symptoms & I have no idea what he was exposed to.

These are just a few things that my son has brought to my attention. I also believe he is on drugs of some sort, I was raised by addicts & his behavior in person is spot on. Also the fact that he locks himself in his room at all times. He’s very short tempered & has unpredictable behavior. What can I do legally? Do I have any grounds to stand on, knowing I can’t prove hardly any of this, just what my son told me? Can I file court papers or anything with police? What the fuck do I do?