If you are "holding" your horse in a frame then the horse isn't strong enough to do it himself... what does that mean? It means your horse has zero topline or a superficial one so you end up building all the wrong muscle due to fatigue, strain and stress. Mentally the horse will go sour. It's your HORSE that carries himself and that starts with correct lunging. "Round" means the topline stretchs all the way through the nuchal ligament and the core muscles actually shorten which makes the horse "look" round. They aren't held there!!!
So learn to let go, i.e. Let your horse stretch his nose forward and out as Shelby Swann described. (And she described it so well and clearly). It's the classical manner - it takes longer than you think for a horse to get strong. I suggest you go through some of Art2Ride YouTube videos. You will only become a better rider for it. 🙂
Try starting him out long and low instead of putting him in a frame right away. Chances are if you're trying to build his topline he isn't strong enough yet to hold himself in frame and that's probably why he's getting grumpy about it. Think about it. A 4 year old baby isn't going to ride in the same frame as a grand prix dressage horse. It can take almost a year to develop a good topline. Long and low will build the topline the same way being in a frame will and it is much easier for unfit horses. What long and low does is brings his pelvis and legs under him and in turn pushes his back up. Once he is there you can start to bring his head up slowly to the frame you want and he should be more comfortable since those muscles were already working. if he start to fuss again let him go long and low and start over with bring his head in to frame. Another great exercise is hill work. This will build his back muscles and help you in the arena. Hope this helps!