Baby Daddy Issues. HELP!

KickFace6

Okay so it has been awhile since I have been here. My ex and baby daddy takes my son while I am at work in the evening. On several occasions now when I have picked up DS his butt has been bright red with bumps the rash is soooo bad. And his diaper is usually extremely full. I am working on building a case against him so I can get custody but I am not sure how to properly document the diaper rash. Do I take him in to a hospital to properly document it or call the police? I just feel at such a loss...please help. If you need back story let me know. Thank you.