5 year old 17h thoroughbred acting up. Help?

My mare used to exactly like that (only in her case, she had been out of work for over 5 years) and while she's still not perfect, she's much better. In her case, I believe it was a saddle fit issue. It turns out that the saddle I was using was pinching her shoulders and basically causing her pain with each step. I ended up just switching to a western saddle and while she still walks off when I mount and has a hard time stopping calmly right after mounting, she never rears/bucks/what have you anymore. It did take a few sessions in the western saddle before her behavior really started to change but it happened.

I would definitely get his saddle fit checked out if I were you. =) It could be (if you're using the same saddle you tried him in) that he's grown/filled out because I believe around 5 is that major filling out stage, right? And now the saddle doesn't fit at all where it used to.

Good luck! And, welcome to the forum! =)

The rearing, if it was a one off let it pass but be aware. Next time he does it, run him full pelt backwards then backwards some more. Teach the brute that rearing is damn uncomfortable. If he gets away with it he may begin to strike and that is extremely dangerous!

As for the problems under saddle. He's a young tb with a new owner/riding. he is going to test the boundaries, that's just what they do.
Keep him forward and thinking. Lots of circles, transitions, changes of rein, serpentines, leg yield etc. etc. Don't go more than a few strides without changing something about how he is going. You need to keep him thinking so that he's more focussed on the next thing you're going to ask than how to get out of the work.

When he bucks, really get up him, force him to go forward and really drive him up to the bridle. If he is the type of horse that you can feel a buck coming on, pull his head around and spin him on a small circle, then ride out again.

The crow hopping is a matter of not having him forward, they can't do it if they are travelling so when he goes to start crow hopping, get up him and get him choofing along so that he cant get the momentum to crow hop.

I don't take rearers. Not into that type of dirt. He can't rear if he's going forward, so keep him active. If he stops and jacks trying to go up, again, spin him and kick him out. It's far better to avoid the rear than get up into a vertical rear where he can flip over and badly injure you.
Horses that get into the habit of rearing, I've heard that cracking an egg over their head at the height of the rear will make them think they've cracked their skull and they won't try it again. I've never had to go to that extreme to 'fix' a rearer, but a friend did so on her ottb and he has never reared since.

You obviously know your horse better than anyone on here so take what I'm gonna say with a grain of salt: my girl will rear when she's confused or when she has way too much energy.
If I start getting all angry with her for rearing, like making her back up quickly or having her do tight circles or whatever, she'll just rear more because she's getting more and more confused. If instead, I just ask her to go forward and just sit on her back, not doing anything, she'll come back to me very quickly. I've been working with her for the last 2 years and the first 6 months was full of rearing almost every ride but once she figured out that I wasn't going to get randomly aggressive when she tried to show me how she was confused, she stopped rearing for the most part. She'll still rear, very small, if I push her WAY past her boundaries but for the most part, she's learned that I'm not going to push her to do things she can't handle and she's learned how to show me she can't handle stuff before she needs OUT. She currently hasn't reared with me on her back in at least 4 months.

I've worked with two rearing horses so far (I know that's not very many, not nearly enough to make a "rule" so I'm TOTALLY not saying this is every rearing horse) and both of them were completely solved with a change of saddle, calm riding, and more exercise. I'm sure there are some horses that just rear to get out of work but I'd bet that's the exception.

Just my $.02.

I agree with making sure he's warmed up too. That's a very good point.

Wallaby that's a good suggestion, however it doesn't sound as though the rearing in this case is caused by confusion as it has happened on the lead, and within the first 5 minutes of the ride where you don't ask for a hell of a lot (I hope the OP doesn't go straight into demanding movements like half pass within the first 5mins anyway!!!). If he is confused by simple walk and halt aids, either the OP is a terrible rider, or the horse is a breaker that has not had very good preparation work done. Neither of which I think is the case.

Also the fact that the rearing is combined with bucking and crowhopping, I was say it is purely a case of a young tb being cold backed and testing the boundaries. And being so big and powerfull, the OP will sure as hell want to get on top of the behaviour before he takes cranks it up a notch.