Not knowing your kid I'll give you credit for making a realistic assessment...
of his abilities and in his case just focusing on the one sport might be his best chance to really accel at one. You're his dad so you have his best interests at heart.
I get that not everyone is the same and many, probably most, are limited in what they can achieve athletically and so they have to make decisions based on that. Nothing at all wrong with that. What I dislike is coaches trying to force specialization onto kids although I do think in some cases maybe a coach has made an honest evaluation of a kids ability and is just trying to help them by telling then their best chance is to focus on only one.
And I don't think that coaches trying to get kids to specialize are necessarily nefarious, just misguided. We all get caught up in the importance of the things we do and are in charge of and give it more weight than the things we aren't part of.