Need advice for student rental property that includes my child

[Rich Waldis]

We bought 2 student rental properties in 2016 where my son and daughter attend college. While we were open to the idea of one or both kids living in the rental property, it was not the only reason we bought them. We intend to keep them after the kids are out of college. We have had renters in them since we purchased the homes but my son is going to move into one of the properties at the end of this month. 

The one property will thus have my son plus two other renters that will sign lease agreements and provide deposits. My question is should my son pay rent (whether I fund it or not) by giving me a check for accounting and tax purposes or is that a waste of time if I am paying for it anyways? Is there anything else I need to consider regarding having my family member live in a rental property that I am treating as a business?