Your son has a difficult temperment. I know that's hard. I had one with a difficult temperment, too. He had tantrums that were embarrassing and hard on him, because he had trouble calming himself down.
This is a long post - sorry for the length, but I tell you all of this to help you. Mine was 8 when I was going through this with him, so I DO understand...He is grown now, so I have the benefit of hindsight, which you don't have. So I'm going to be straight with you. Stop feeling sorry for him. If you do, he will know it and he will USE it against you.
This same son of mine, as a college kid, worked at a camp on a college campus as an RA. He had a group of boys under him 24/7. One of the kids, a 7th grader, was a holy terror. He was in trouble all of the time. It was a serious thing to send him home because like him, most kids flew in an airplane to get there and it was a month-long camp, which cost A LOT of money. And there were NO refunds for kids who didn't stay the whole time. Right before they made the decision to send him home, they asked him what he thought needed to be done to get him to understand what he needed to do to behave. His answer was that they not put him in positions that made it so that he lost his temper.
I'm not kidding. This kid was telling them that it was everyone else's fault that he was so badly behaved, and that it wasn't his own job to control himself.
The reason I tell you this is that you don't want your son to end up with this attitude.
I loved my son, but I gave him tough love for bad behavior. How else was I going to get it through to him that he couldn't act like this? YES, I did keep him out of some activities because they were triggers, but not to the extent that this kid had, and certainly not at this age. I would take him to activities that were problematic, after telling him before getting in the car what I expected of him, what exactly would happen if he didn't toe the line, and then do EXACTLY what I said I would do. Sometimes we would just turn around and drive right back home if he started getting upset in the car. Or we'd leave an activity very early. Or I'd tell him that there was no more of it for a week or two until he could decide that he was going to behave better.
It took a long time for him to grow out of this. As hard as it was on ALL of us, I kept at it. He's a terrific young man now. I'm so grateful that I didn't give up.
With boys, you don't talk them to death. And when they're having a tantrum, you put them in their room and leave them until they straighten up. And you NEVER give them what they want when they act like this.
As a example, my son had trouble playing putt-putt and bowling. The more he would lose, the more upset he would get, and the worse he would play. He had a perfectionist streak in him that was really awful for him. Did I feel sorry for him? NO. I wasn't going to let him have a tantrum over these two games and make everyone else around miserable. So he only got to do them once ever so often until I knew he had grown out of this. And each time I gave him the low-down that if he lost his temper, he would go home and lose privileges AND not get to play for months. That's how serious I was about this. Eventually losing didn't bother him and he started enjoying the games. But with his serious personality and temperment, he couldn't see it. I had to DEMAND that he see it and be hard on him to get it through to him.
This is what you must do if you want your 8 year old to be done with tantrums by the time he's 10. It's really ugly when a big kid has one. I have a friend whose grandson was 12 and doing this sort of thing that MY 8 year old did. Awful...
Bike riding can't be the only thing you take away. You have to have more of a toolbox than that. TV, computer, gameboy, Ipad, anything BUT books. You are doing the right thing by him getting plenty of sleep. My son could not manage himself without getting at least 9 hours of sleep. I couldn't keep him out or up late or he would have a big blow-up over something the next day. Every time.
If he has a tantrum before school, you put him in the car, drive him there and push him into the school without ceremony. You should have a frown on your face and tell him that you won't listen to his complaints. His job is school and he is to get to it. Don't bother telling him what privilege he has lost - you can tell him when he gets home. Make sure that every day when he walks in the door that he washes his hands, eats a snack and goes to his room for 15 minutes of downtime. If he has lost a privilege from earlier, THEN tell him.
Put away ANY clothes that are no longer in season. If he can't find his shorts in 30 degree weather, he can't have them on for you to tell him no. And yes, you get to have that power in the relationship with your son. When he finally figures out that any THING that he fights about, whether it be clothes or toys, disappears, he'll eventually stop fighting about it. You don't talk about it. You just do it and he will learn the hard way.
I hope that you can soldier through this, mom. I know it's hard. My son is still very serious, but a terrific young man. Stand firm, and your son will become one too.