I recently experienced something similar. I went and looked at my mare, though I wasn't looking for a beginner friendly horse, I was more looking for the next step. I tried her out three times, despite her having been off as a broodie, she stood stock still for mounting, moved off quietly forward when asked. She looked at things but didn't spook, she had her head in the air some of the time, but she's half Arab and not muscled up for carrying herself, so that was not a concern for me. I brought her to a boarding barn near me for a 30 day trial. I was so confident in her and she settled in so well that I was riding her the next day in the indoor with no problems.
Gradually, she started spooking on the spot under saddle, then she'd spook and run a few strides. The worst it got was half a circles worth of running, so no true bolting, mostly she would spook on the spot if we were walking or curve her body away from "whatever" she was spooking at and cut across a circle or whatever to get away from it, but without really increasing pace. Then she started to not want to stand still for mounting, or she would try to walk off and start jigging after I got on.
On the ground, the first time I was washing her hind legs she was fine, then after the spookiness started, she didn't want to be washed or even in the washing stall anymore when previously she had been fine. Other things, she would move away from me when I was tacking up, she would move into my space, she started to get really buddy sour, did not want to be out of view of other horses.
The problem was that I was letting her get away with little things because I wanted her to "love" me, and forgot about the whole respect thing. She is my first horse, and though I'm no stranger to horse handling, it was always other peoples horses' so it didn't really matter if they "loved" me as long as I could handle them safely.
So I researched some ground training techniques, got some help from my coach, and started trying to be much more consistent with her. Yesterday was the first time in awhile that she did not even try to move while I was mounting, or move off without instruction to do so.
Anyways, my point, is I see in your pony's behaviour a lot of things I started to see in my mare. You need to be firm and consistent with handling this pony. Also, if he's rearing, I would definitely consider not riding him until you can get a professional to at least look at him. In the meantime, look up some ground training exercises, and work on being absolutely consistent when working around him. If you tie him up to groom him, make sure he stands quietly until you are done. Pick up his feet. If you are feeding him yourself, ensure he does not get access to the food until you say it is ok, and carry a crop with you if that is whats required to keep him from mugging you.