My son is 10 and does quite a few chores around the house. He does the same thing! It's almost funny- the thing with feeding the pets, but forgetting the water. Or unloading the dishwasher, but forgetting the silverware... I really don't think he is slacking off or trying to get out of work. I do believe he honestly forgets!
My theory is that his brain is growing so much right now, absorbing and processing so many things, etc. that when he isn't really focused specifically on something (especially a task like a chore, that is just routine and doesn't require the type of attention that something like a math problem would) his brain has just already shifted to something else before he finishes. From talking to other moms, it also seems like this happens more with boys, but that might just be my perception. I don't believe this forgetfulness automatically means he (or any other kid) has ADD. I think it is part of the growth process.
The easiest fix we've found for this is a list on the fridge. We have a morning list of things he has to do before leaving for school (brush teeth, hair, feed dogs & cats, don't forget bookbag, lunchbox, oboe, etc) and then another afterschool list (call mom,let dogs out, set up homework on kitchen table, get snack, etc.)
This might seem over-detailed- but because its just posted up there, he only has to remember ONE thing: GO READ THE LIST! If I am not home, he can refer to it immediately, if I am home, it means *I* am not nagging him to do something or telling him, etc. he is responsible for it himself.
We don't do checkmarks or anything like that- just the written list to read. It works for us- maybe it will work for you too!
For school, his teacher gave all of the kids assingment books, and that has helped a LOT. Everything he needs to do each day is written down, and a parent is supposed to initial it each evening. That way if he misses something, I can ask about it. Also, our school now has ALL classroom assignments and worksheets, etc. available online through their website. So forgetting a sheet or homework at school is NOT an excuse- we just download it!
I bet your school has something similar- if not, you should ask about it, because I bet money that your son is not the only forgetful 5th grader out there! :)