I tend to emphasise the work and dedication needed to get to where/what my DS wants to be.
He can ‘be anything he wants’, but some anything’s are more realistic than others. Some will require considerably more effort - and potentially talent/luck/connections etc.
In some cases, he’s clearly never going to get there but there are other things he could do instead. For example, telling him he should set his dreams on becoming an astronaut is unrealistic. He might have a better chance at becoming a rocket scientist, or aerospace engineer, or an astronomer, or a commercial pilot or a whole range of other things. But all of those options would still require significant effort and dedication to get there. And there’s lots of competition - some of whom started off in a better position than he did.
I realise that there can be a whole range of barriers that hard work does not necessarily overcome - but the most useful thing I can tell him is really to put the effort in and be a bit flexible about exactly what he wants to be, while recognising that you aren’t always going to be the ‘best’ in any situation.
He gets it, not least because he really enjoys sport and has plenty of experience of doing loads of training and then competing against others. So he’s learned that sometimes being the best is related to things beyond your control. The oldest and tallest kids in the age category tend to outperform the younger ones, but that all shifts at the next competition when the oldest kids have shifted into the next age category. You can be the best in your local area, but move up a level and the competition is tougher. Sometimes, you find kids that have basic advantages you just don’t have (physiological or in terms of support).
All you can do is work hard, try your best and enjoy doing as well as you can. Wanting something doesn’t just make it happen, but being willing to
work at it might increase the chances that it will.