why am i suddenly getting my sons text messages on my mac

This will happen if he is using the same Apple ID for iMessage on his phone (in Settings>Messages>Send & Receive) as you are using on your Mac, and you have selected his phone number to receive iMessages at.

To fix this, go to Message>Preferences, select the iCloud account, then uncheck his phone number under "You can be reached for messages at". This will prevent iMessages addressed to his phone number from being received on your Mac.

You're welcome.