When you play against him, use all of the time that you would normally use in a tournament game. Leading by example is the best way to teach.
How to slow down the pace?

He's 8 years old. Of course he's going to play fast. It's an achievement that he can play 20 minutes straight.
That said, you can create a checklist of things to do before making a move - when practicing, maybe even make it a physical list that he has to check off, and before a tournament remind him to think of "the list."
I'd suggest involving him in the creation of this list, ask him what he thinks are important things to look for before moving, and any basic things he doesn't think of, ask him, "do you think x is important?" This will help him to appreciate that this stuff really is important and he will be more likely to incorporate it. This way it's not telling him to slow down, but teaching him why he should slow down.
That, and the "sit on your hands" technique seems to work for both kids and some adults who have problems moving too quickly.

The way I slowed down is by starting at 10 minute games and gradually adding 5 minutes at a time. When I was playing to fast, I sat on my hands and decided to create a pre-move checklist.

Thank for all your suggestions. Luckily he is not stubborn or adamant. He understands the value of playing slowly, but like you said it is his age. He gets excited and rushed to play, I guess, at that time.
I asked him to get up from his seat once every 10 - 15 moves and take a stroll for 15 seconds or so, which may relax him so that will control his excitement and rush to play. And he started doing that since last week.
Your suggestions of leading by example, sitting on hands are good. Thank you.
I am also trying to use games vs Deep Fritz, where we have the option of looking how much time he took for each move. I set the minimum amount of time per move as 30 seconds. I might even increase that to 1 minute if it helps.
Thanks again.

Depending on which time controls he is playing, it could be reasonable to use 20 seconds for a move, or 20 minutes! It all depends on how serious he is about playing well.

My son used to do tactics in the past but not regularly. To be honest, I must say we neglected on tactics in the last few months. For the last few days, he is doing tactics which includes more than 2 or 3 moves. He is making less blunders now and moving slowly. He still is making blunders but reduced considerably.
I guess when you do tactics, obviously you slow down while thinking and trying to solve. If that trait can automatically be used in the games, then players like my son will find more success.
:)
Everyone of us are sure that we need to play chess games slowly to maximize our potential. My 8 year old son is rated ~ 1000 USCF but I have not seen him play a game over more than 20 minutes. Yes, he slowed down a lot but still, this pace is not helping him. He started learning chess last summer, started playing scholastic chess tournaments since Nov. 2013.
I do not encourage blitz games at all, I never allow him to play fast games. So, that is not where his habit started. The scholastic tournaments he plays have a G/40 setting. Yet, his games are over in less than 20 minutes.
I play chess as well, I do play OTB games with him at home but he always takes less time than me. I even stopped teaching him any chess, threatening him about his fast pace but that doesn't help any. Whatever I talk, once the game starts, he is at his own pace while I watch amused.
I don't mind him playing too slow, even if that means he loses. I would take a game lost on time, which is fine by me.
What should we do to stop him from playing fast? Any suggestions? Please.