If it were my child, I would say something along the lines of "it is your choice where you go to school, but I hope you trust me as your parent to guide you in that choice. School A will offer you these benefits, School B will offer these other benefits. No one can tell which school will ultimately be the best choice, and you would be doing yourself a disservice to not keep your options open, for as long as possible. Who knows what you or I will think in 12 month's time, best to do your best and give yourself every possible opportunity. Sit the grammar test, do your best, apply for all the schools and then you might have the opportunity to choose between those schools. Don't try and the choice is made by circumstance, not your own choosing."
A bit long winded! But very similar to what I have said to my kids in the past...
And are you discounting the sports school for any particular reason? Will it be the back up school if your son doesn't get into the grammar?
Having qualified for the grammar school but before you complete your CAF is when you have the choice of which order to rank the schools. If he does well enough in the 11+ to be assured of a place at the grammar school, but would seriously be happier at another, please don't just put the grammar school in first place on your CAF without explaining to him that the first school for which he qualifies is the one that he will get. Don't let him think that he has an easy further decision point after allocations day, when there is not necessarily any chance of him being able to get a place at a lower-ranked school.