Thank you all for your input!
Some great suggestions. (Orange balls out of closet already for example)
My challenge is basically that I see he has more natural talent, killer instinct, to dominate at the net in doubles (than run from side to side 5 hrs in singles) no matter what agility practises and miracle workouts - but how can I keep that up, encourage him on that, when there's no way to dominate at the net against his (good) practise partners - against whom he can compete alright from the baseline. Unless I make the court 2/3 or something, and who wants that? of course by drilling but in competive play?
So basically I see the future so that he can and propably will become a very good singles player (but THAT good, no pro potential unless future proves me wrong) while in doubles he might have a shot if he decides. And If I'm innovative enough.
How to really maximise that?
By the way, I'm just curious, what you guys think is more difficult, atp top 10 in doubles vs top 100 in singles? (Of course it's a matter of talent but in general)