OK is what is known as a "1-party consent" state. What this means that as long as the person recording a conversation is a party to it, then the recording is legal. This menas that if the person is not a party to the conversation, then recording it is illegal. The law does makes an exception for in-person communications when the parties do not have a "reasonable expectation of privacy". So for example, it would be legal to record a conversation in a public place where it might reasonably be overheard, even if you were not a party to that conversation.So depending on where you record matters. If it is within the therapist's office, you would need to be in there with your son in order to lawfully record. If it is in an open area of some sort, then they have no expectation of privacy so you would not need not be a party to the session in order to record it.