I would let him know what you think once or twice but leave him to his own choice.
Nobody wants someone suggesting to them what they should be buying and carrying more than once.
If he has read up and handled both calibers then he'll be able to make his own decision.
You can always change your preference later, it's not a life sentence
I bet the majority are saying 9, get the 9, dont be an idiot do the 9, dude 9,bro 9, trust me get the nine..
Oh for fuq sake shove the nine up your ass and i'll choose the bastard 40
I remember thats how it went for me hahaha
Anyhow a father would always like the best for his son and i'm sure you mean well
And now being a father myself we'll see how i try to impart my knowledge/preference when that time comes