As others indicated, this is a good life lesson for your son to think about and decide for himself. Guide him in thinking about all the factors involved and possible consequences or opportunities arising from his decision. There is his commitment to his team - if he chose the "elite" team, could he live with letting his teammates down (regardless of if they win or lose)..would the act of appearing to "dump" his team this once give him a negative stigma, would his current team's coach and fellow teammates understand what a great opportunity playing with the "elite" team is and be understanding and encourage him to take up the offer...
Is the offer to play on the "elite" team a one-time thing or on-going? Is it an opportunity to sub for a regular player who was not able to make it? If he chose do decline due to his prior commitment would this look favorable in the eyes of the elite team, that he's a committed player, and by not playing with them show character that they may ask him to play again in the future? Would the team think negatively of him if he chose to honor his commitment, thinking he's not ambitious enough by not dropping his team - if so, is that really a team attitude he wants to play on?
There can be many more questions to ask - I was just brainstorming not knowing all the details you probably have. Be his councilor in this situation, and support him in which ever decision he makes.
If he's as talented as you state, this is only the beginning of choices he's going to have to learn to make as he continues on in his career.