Actually, kid can start as early as 2. Just give them a 19" stick (like a padel pad). then throw them ball, or even hanging a ball with a string with a stick for them to hit. Give get the feel of the ball contact.
Good to hear that. It's really good to have your kid starts early. As at the age of 7, his body structure and build is already growth to certain level, which allowing him to have good and solid contact on the ball. During this stage, his learning curve should be very fast, and able to listen to instruction and have reasonable hand-eye coordination.
Now lots kids started play tennis every early like 3 or 4 years old. I been teaching lots kids like this age and have good potential. But i would like you know for all young players at beginning please don't just focus on how many time they can hit the ball. The most important job is to build up every good common sense. That's include moving, catching, balancing, and have lots fun which them. So next time they can bring you to the tennis court not you bring them back. For more details you can contact me at<firstname.lastname@example.org>
2 years old just little early to find out a coach. First he can not concentrate about 1 hour, Second he got tired every easy even the group lesson.And then you just throw money. if you have time try bring your kid to tennis court or any where have enough place got some soft ball and kids racket play with them together. Point is let them like tennis.