Your child is shy and under confident.
Your DP knows this.
Your child is asthmatic and a poor swimmer. Your DP knows this.
Your DP physically overpowered a child and inflicted an emotionally humiliating and life threatening act on him.
Your child was emotionally and physically distraught.
Your DP saw this and chose to add a further level of humiliation and abuse by claiming it was a “joke” implying that your child was being a wuss.
He did not attempt to rescue, soothe, calm or apologise to your child for his “joke” - instead he turned it all around and attacked you adding another level of emotional trauma.
Your DP is a thug.
His actions are indefensible.
Your child already has had a v tough start in life by being rejected by his own father - he needs extra nurturing (as evidenced by his confidence levels) - he does not need this thug in his life.
He needs his mother to protect him.
How long has this guy been in your child’s life. How has their relationship been to date?
Clue - nothing less than kind, respectful, encouraging and supportive is good enough.