Has he got any sporting heroes who might have spoken in an interview about the amount of work/training they have put in to get where they are? A few have done documentaries that really show what is involved and if it is someone he admires it might have an impact - especially if it shows the sports person struggling with a particular skill and how they go about tackling it.
Alternatively, is he the kind of kid who will sit and listen to reason when he is calm/happy etc? Maybe present it as an experiment to him - pick a skill in whatever activity - nothing too difficult, but something that he has struggled a bit with - maybe something that previously got him frustrated and triggered the subsequent sulk. See if he will agree to work solidly on that one skill for an afternoon (for example) without sulking or quitting - just to see what happens.
Just focus on that one skill - chances are he will improve in that afternoon - so that will demonstrate how the improvement comes - and probably give him a confidence boost. Maybe a bit of bribery if he sticks with the practice session and sees it through. The point is to get him to stay long enough and do it long enough that he sees improvement in that one session. In his mind he's thinking "I can't do it! What's the point?!" - he has to experience pushing through the challenge and realising he can do it - just because it's hard, doesn't mean he can't do it.
If he's doing lots of different things and trying lots of different skills he may see the practice involved in getting good at all of it as too much - the goal is too big - too far away. If he has an impatient personality he needs to see quicker results to trigger his motivation. So you need a small goal to start with so he experiences the process of achieving it.
If he improves one skill in a day - doesn't have to be perfected - but if he goes from say - unable to serve in Tennis so the ball goes over the net - to actually getting a few over the net - and maybe into the correct area - then he sees the progress immediately.
Narrow it down - pick a skill that you think he can improve on in a few hours - even a bit - so he can see the results. Then suggest another skill another day - so it starts to build up.
I'm no expert in this at all btw - but I have an impatient personality myself. I struggle with a few skills in a sport I do - had a mental block. I recently started just taking one skill and working on it for an afternoon - breaking it into chunks within that session, and got great results. It's really motivating. Sometimes I'd say "okay - let's do this for an hour and see where we are" - usually I'd already see improvement in that hour and I'd end up there for another couple of hours.
Thinking back, when I was a child, no one actually taught me HOW to practice things - how to break stuff down and do little bits - perfect them one at a time - then build it up. Often kids are told to "practice" but they may not understand how to really do it. I find it fascinating watching things like gymnastics training/practice and seeing how they break down skills/moves in order to perfect them. Maybe a few YouTube videos that show skills being broken down like that might help so he can actually see that they don't all just turn up one day doing all these tricks - there is a process to learning them and perfecting them.
Don't know if any of this will help but maybe worth a shot!