Perhaps not necessarily wrong, but people sometimes don’t like hearing the truth. I don’t agree with the other comments that you should support him - to me, that sounds like bad parenting to not at minimum set up a backup plan when things start to fall apart.

My take on this situation is that you may have to tell him in a more subtle manner. Fortunately, professional athletes are known for being notoriously competitive, where even talented people can lose out on an opportunity simply out of bad luck. This could give you a way to voice your concerns in a way that doesn’t directly imply that he’s not cut out for the job, ie “you still need to be very lucky to go pro. I was very fortunate, but we shouldn’t be counting on luck for your living. You can still try for it, but I really think you should also have some backup options if things don’t work out”