I agree with SC310, at 7 yrs old if he has a swimming background, he is already ahead of 75% of the 7yr old new polo players. It’s important right now for him to have fun and say “yes this is something I want to do” then as he grows and you start going to tournaments and start seeing the different clubs and how their parents and coaches act and interact with the players, and the opponents, it will help you guys decide where he may want to keep playing.
If you start overthinking it right now and start travelling far because you think that’s the most competitive club for a 7 yr old and his development, he might get tired of the long hours and decide he does not like it and may want to quit after a year.
As he starts playing more, you will see where he belongs and where he does not.