It is better to make his own passion on swimming and then his own passion will let him do it. It is true that he needs to learn swimming for sort of life in future though but his own mind set up is important though.
Like you are with kid at his very little age and playing with water,which bring him interest in there as you are with him there for the time, so it is better if his dad will be the time in water too.
Personal training is much more effective and fun though.
Convince him that if he learn swimming earlier ,he can sleep more at holidays.
Best wishes for the kiddo.