Maybe he's sort of "re"-testing you out? You've been back for long enough that perhaps the honeymoon phase of "OMG! My mom is BACK FINALLY!!" is over and he's into the "Huh, she's a better rider now....I wonder if she'll be easy to push around or not.."-phase..?
Did you have a crop with you?
Lacey used to try those tricks on me [she has gone as far as rearing though - her previous owners would just get off and be "done" when she'd do these things so it was a pretty ingrained habit] and I found that the one way to get her out of it was a solid smack on the butt with the crop. I'd give her plenty of rein in front and let her go forward as fast as she wanted, for a step or two. For her, rearing, and all that, was simply evading going forward [undersaddle, she would ever dreammm of rearing o the ground] so it was a "easy" fix. Some people advise getting off to deal with it, but I don't know how I feel about that. At least for Lacey, the rider dismounting IS her goal so, even if she's worked on the ground, it's still more rewarding than being put to work undersaddle.
I do continue to ride with a crop on my saddle, just in case. It was such a ingrained habit when I got her that I doubt she'll ever be 100% over it. We've gone 3 years with 0 rears, but that's because I make sure to diffuse the situation [via well timed crop "encouragement"] before she gets to rearing.
Of course, I have much higher hopes for you and Sky - unlike Lacey, no one's taught him he can do that sort of thing and get out of work!!
I'm sorry you had issues.