Sam Warburton's got some great stuff to say around this, as a successful state school sportsman (albeit a state school which had good facilities and support). Look at other successful state school professionals in your son's sport for their inspiration and advice, look at their autobiographies.
For some nuggets from Sam Warburton that I've shared with my teen son on instagram:
"it takes a certain person to get to the top" "people think it's just talent that gets you there alone. Talent's great, talent's the catalyst that you might need but if you're not willing to work.... mate I've seen so many guys, I wasn't the best guy in Cardiff growing up but I was just the hardest working and the most committed and I was willing to do it for the longest amount of time and if you're just willing to stay in that fight and that grind for a long time you just beat so many people from work ethic alone... there's a number one denominator, key denominator to all successful rugby players from when I've been in changing rooms with them and it's resilience mate, they just all keep going, refuse to give up, refuse to be told no. And no one has an upwards trajectory, I think people look at pros and think they've had it easy, that they've always been selected, they're always on this upwards trajectory all the way through their youth career all the way through to senior but it's not. They've all got bumps but they've all just had this stubborness and resilience to keep going and that's why they reach the top so when you're in a dressing room with all those lads it's a pretty powerful place because they're pretty tough. And I don't mean fight tough I mean mentally tough and that's a great place to be."
Sam's also talked about how far ahead young players can get if they're willing to do what other players aren't with regards to number of hours in the gym, attitude to training etc. Not expecting the world to fall at your feet. If you're willing to put in 2 years when no one's looking, like if you're dropped at 16, you've got 2 years to build your athleticism when others might not. "The common mindset I've seen in everyone who gets to the top, when you're at school hard work is not really that cool. But when you're in a pro environment, hard work is pretty cool."
I can absolutely understand where you're coming from and maybe he won't have the career he hoped for but there's some excellent advice and mentorship out there from people who aren't from a more privileged background.
My sons also go to state schools, they both love rugby. They're not pursuing it as a career so it's not as pressured but they do want to be the best that they can be so they can enjoy playing decent club rugby alongside careers in other fields. They've had far fewer opportunities to play for their school than their friends who are in the local Welsh language school because there are fewer boys interested in doing the sport.
As well as sport specific inspiration and support, I'd also recommend having a look at the work The 93% Club are doing.