There are a lot of things that I agree with here. I would definitely have him checked out for pain. I would be getting a good exam done to make sure nothing is wrong.
I, too, beleive respect is a two way street. And IMO respect is not showing a horse who is boss. Respect is appropriate response to pressure. I have to respect my horse's nature....to be a prey animal and to fly from fear, to be perceptive to people, places, changes and things, to be grugarious. I have to respect his thoughts, feelings and opinions otherwise I am just being a dictator. At the same time he needs to respect my space. He needs to know that I am alpha in this herd of two, but I need to show this to him in a way that he understands and goes along with the nature of the prey animal in order for him to accept and respect it.
I would stop with the round pen work. IMO round penning has ruined more horses than it's helped because people just go in there and run the horse around, sometimes "teaching it a lesson." Horses live in the moment, they don't associate round penning with an action they did several minutes ago. If you are going to correct a horse it has to happen IMMEDIATELY otherwise you lost your chance.
It sounds like when he spooked he wasn't trying to kick you. Spooking is a fear response so I don't think he intentionally kicked you.
As for in the round pen, this horse is only 4. His play drive is probably very big so the biting may have been him trying to play with you or dominate you. But the biting may have been provoked...what were you doing when he bit you? Horses ALWAYS give signs they are about to kick, bite, strike, etc. You just have to know what to look for. Just remember a horse can't bite you if he's not close to you
I'd be teaching this horse to back up on-line while I keep my feet still. Get him out of your space. The kicking out could be he was just playing or because you were too strong with your correction. The correction may not have been fair and he told you so by kicking out. Or it could be that he was saying, "I don't wanna!" and throwing a little tantrum. If a horse throws a tantrum like that when I'm working with them I just ignore it. I just say, "That was pretty but please do what I asked." If he does, great, if he doesn't I'll do something about it. But throwing a tantrum like that is not respect....he's not responding appropriately to whatever pressure I put on him so it's my job to fix it, not punish him for his opinion.