I will preface my comments by saying that I have no experience with stallions and can’t address that aspect of the question. But I do have a young Holsteiner gelding (no idea on the blood percentage, but here’s his pedigree - http://www.allbreedpedigree.com/campiro) who was pretty quiet from the beginning. To the point that I was worried he wouldn’t “wake up” to the bigger fences (though he rarely touched a fence), but nothing too out of the ordinary for a quiet horse. Did the 1.15m in his 4/5yo year and he came home from the last show of the year with a bug (fever and cough that wouldn’t go away). He got lethargic with the bug and it just seemed like he never came out of it. I suspect that it was a coincidence, but it’s hard to say. His 5yo year was awful - it was a huge battle to get him to go forward at all, and then the beginning of his 6yo year was terrible too because he added a spooky attitude to the deadness.
We ran blood with nothing unusual showing up and I also tried gastroguard with no changes in his behavior.
Now heading into his 7yo year I finally see a light at the end of the tunnel. He’s started to “fire up” around a course and starting to respond under saddle with a bit more oomph. In the process I’ve had several people tell me that they’ve been through the same issues with their WBs in the 4-7yo range.
I will say that my guy was a relatively slow maturer. In fact, at coming-7 he’s still super narrow through his shoulders and clearly has more growing to do. But the majority of his “going forward issues” coincided with the process of his hind end growing and maturing into something that fit his body. I think I would have saved myself a lot of frustration if I would have just tossed him out to pasture after his stellar 4/5yo year and left him until this year.
In regards to the hot versus not attitude for a jumper, though, I will say that my personal preference is for a slightly dead and extremely smart horse. My best upper level jumpers have had that combination of dead-type and wicked. I definitely feel like I’m in the minority, but I do know a handful of BNTs who feel the same.
So I guess my comment would be that a) I wouldn’t put too much concern into attitude until he’s done growing and b) I wouldn’t worry about “type” (dead versus hot) until or unless your trainer actually runs into problems around a course because of the attitude.