My phone's iMessages are being sent to a the intended addressee. However in addition to him, his father's email account just recently started receiving our texts last night. How can we stop him from receiving our texts and replying in our thread?

I've tried turning iMessage on and off, his son has deleted his iCloud account in suspiscion that that was the cause of the problem, etc.

We've been texting for months without problem. This morning his son texts me, "Good morning," and then his father replies into our thread, "Good morning to you too!" I checked my contact, and I never had his father's email account before nor was his email a part of his son's contact. Please help me to stop him reading our messages. Thank you.