My personal opinion is you’d be far better off climbing the ladder slowly and moving up to TaG or something, but only once he’s mastered Yamaha. Meaning being a front runner regularly. As soon as you move up, you’re handed more variables to deal with, especially with a shifter. Even moving to TaG, you’ll have to develop a little different driving style, learn new chassis tuning techniques and the competition level will be higher.
I’m always an advocate of moving up classes only once you’ve shown you’re capable in the class you’re in. Once you plateau, then move up. But he’s your kid and it depends on what your/his goals are in karting. If it’s to eventually be a top-level driver, I suggest moving up slowly and learning each step of the ladder. If it’s just to have fun and drive something fast then you can do whatever you guys want.
If you want to go shifter, it’ll probably be fine from a safety standpoint, just make sure he has plenty of practice before racing. Stuff will be happening exponentially faster and he’ll be far more busy in the kart so it’s a lot to take in and get used to. The leap from Yamaha Jr. to a shifter is immense. It’s like just learning to crawl and then challenging Usain Bolt to a sprinting contest.