Okay, at the risk of sounding creepy, I'm going to post this anyway.....
Even with the Dis search disabled, a quick google of the OP's username will take you to a pre-trip thread posted by the OP where she names all of her family members. From the that thread and this thread alone, there is ALL sorts of personal info posted about the OP.
From there a quick search on Facebook will pull up a picture of the BIL she is worried about. The picture shows the boy eating a cookie.
To find all of this took less than five minutes. Now, if a complete stranger on the Disboards is able to do this, how long does anyone really think it will take the OP's MIL to figure out the OP made the call to CPS?
It is so incredibly easy to come on to the boards and ask for advice and even easier to give advice based on information which may or may not be completely accurate. The thing we all forget is that there are real people behind these computer screens, with real issues, with real consequences.
OP - I am sincerely not trying to be mean, creepy or anything else and what I'm about to say comes genuinely comes from the a place of concern.
I have no doubt and you are a caring person who is genuinely concerned about your BIL. I have no doubt that your BIL has some issues. Before you make a phone call to CPS which may or may not result in good consequences for your BIL, please, please talk to your DH, talk to you BIL, talk to your MIL, talk to the school.
The call you make to CPS, as helpful as it might be, WILL ALSO come with unintended negative consequences and damage to every relationship you have with every member of your DH's family and these are people who will be in your life forever because of your son.
Before you make a call to CPS, make sure you have exhausted EVERY other alternative in attempting to help you BIL.
The internet and message boards are a wonderful opportunity to gain unbiased advice about a situation but it is very naive to post the kinds of personal info that you have posted and then post a loaded question and believe that it is anonymous.
I wish you, your BIL and your family the best of luck in resolving this situation in a way that causes the least stress to your family. Good luck!