He's being a two year old. But he needs to learn that that is not acceptable. Although his behavior likely, is not malicious and he's playing he needs to be put in his place. With babies, they rear once or twice sometimes to 'take it for a spin' and see if it's going to fly.
Once he rears it's too late to stop him, but when he comes down he needs to be worked and worked hard immediately to make sure he connects the rearing with the work.
No disrespect to you 45 degree angle folks, but I don't like doing that with babies. Some of them are lucky to operate all four feet at the same time, that could be a disaster.