Your son sounds much like mine. Mine is very bright...high iq...is in the gifted program...BUT...has ADHD. Certainly I'm not suggesting your son has the same thing, but you may want to do some reading on it...see if any applies and have him tested if so. It manifested itself in our son in 4th grade.
So...we've been through some counseling with him (and he's had some on his own) to develop some tools to overcome his challenges.
The big thing the psychologist stressed is that he needs to suffer the consequences of failing to follow through on what's expected of him. She reminds us that the stakes are so (relatively) small when they're in elementary school. If you continue to cover for him, he will never learn. He'll be 30 - with a wife and kid and get fired from a job when he forgets something and he'll never know what hit him.
Believe me...I speak from experience. It HURTS to see your kid mess up...and do it again...and again. It feels like it would be so much easier to bail him out, and it would in the short run, BUT...if you continue to enable him, he'll continue to rely on you.
Just remember...the stakes are small now...I have to remind myself of that on a daily basis.
P.S. There are still SOME times that I bail him out...but they're less freqent...and it's for extenuating circumstances only. I also take away his video game time which is very painful for him...the important thing is to be consistent. If we say we're going to take away his game time if he forgets, we do it...even IF...he has a friend coming over or whatever else...