Royal Sporting House should have them. They have branches everywhere - Shatin, Kowloon Tong etc.
The better brands are "Head" and "Prince" for tennis.
My husband and I are both tennis players. We've already bought a small one for our little girl who's only 3.5 years old - from Royal Sporting House. DH is planning on teaching her when she turns 5. Wishful thinking, huh, but DH LOVES tennis.
If you are getting a tennis coach for your son, you could also ask him to recommend one for your son.