We live in NB and have done multiple leagues in soccer, baseball, basketball. I would say age 4, go with i9. Like Agdoc said, its 1 day a week commitment and I feel like it actually gives the kids a chance to apply what they learned. You practice 30 min, then immediately have a game for 30 min. Then next year play with NBYSA or NBLL. My son did i9 age 3/4 and then NBYSA for age 5 to current age 8. My daughter is 5 and doing i9 soccer because her interest is questionable, but she wants to try.
Also heads up, NBLL is competitive and gets intense quick. My son dropped out to stick with soccer/basketball. NBLL once they hit age 6 or 7 starts to have practice 1 day, hitting practice 1 day and games up to 2x/wk. Not all is "required" but of course your kids falls behind the team if you dont try to keep up. Frankly, 3 to 4days/ week was not what we wanted to do at such a young age.
I would also start with soccer at age 4. Works on some coordination, they are constantly moving, and learn the fundamentals of being on a team. tee-ball at that age can bore some kids because of less action. He will be just fine and not "behind" if he starts teeball/coach pitch next year.