Hindsight is always 20/20, but......
If this ever happens again and your son comes to you visibly abused (bruises or any kind of marks on the body that would suggest abuse), call the police ASAP. You HAVE to get this reported as soon as possible in order to make anything happen. Right now, since he has no visible marks of abuse, it might be a waste of time to file the ex parte now, as the evidence of abuse is gone.
Police are mandatory reporters and if they observe signs of abuse they are required by law to report it to social services. But the key here is immediate reporting by you if you observe it.
Back when DS was 4 (he's 22 now) and his dad was still trying to discredit me and take DS away, he reported me for abuse. And he went straight to DHS to do it, claiming I had beat DS. But when they checked DS out and found nothing, BF said that it had supposedly happened 2 weeks prior. They totally discredited his report, but that didn't stop him from taking off with DS 1800 miles away to CA. (That's another LONG story!) But he did leave me a note stating that he had reported me to DHS and to contact them.
The conversation I had with DHS was quite interesting.......they said they did interview BF and observed DS. But when I asked them what would happen next, all they would tell me is that the report would be destroyed after one year if it is unfounded....very pointedly, I might add.
Some time later, I talked to a social worker who divulged to me that they never took BF's report seriously from the get-go, since there were no obvious signs of abuse like he claimed. Their claim was that if he had really been concerned about DS's welfare, BF would have reported it immediately, not 2 weeks later. That's a huge red flag for them. So they had told me up front that the report would be destroyed after 1 year to let me know that they weren't pursuing a case.
I never heard another word from them about this until one year later....true to their word....I received a letter from them stating that the original report had been destroyed.
Always report suspected abuse immediately....regardless of the circumstances.