Hi, B.!
I can totally relate with your situation as a homeschooling mom and mother of a daughter with similar problems (actually the whole reason we started homeschooling)& younger son who doesn't. I do agree with other opinions here that you may want to look into the ADD/ADHD diagnosis. From the time she was around 5 y.o. we started noticing the same things you talked about. She would forget what she was doing just walking across the room! Learning anything in school was a struggle - what she grasped one day was completely gone the next. When she entered 1st grade, her teacher agreed that something needed to be done, so we went through the ADD/ADHD testing. We had her on medication and it helped, but she still struggled with school. That's when we started homeschooling - I've used her interests (art, science, animals) to boost her skills in other areas. We don't do "memorization" exactly - more hands on stuff. She's only recently (at 12 y.o.) begun to read fluently and still has real problems with spelling, but is an avid learner now, seeking out information. Could be, too, that your son is a more tactile learner.(?)
Something else you may want to consider - food allergies. Thinking along the "red dye" theory, we did an Elimination Diet with my daughter. We found out that wheat greatly effects her ability to focus! (And I was feeding her 100% wheat bread thinking I was a good mom! LOL) There are wonderful books on this process - it's a bit of a pain, but it has done wonders for her!
Hope all this helps - sorry it's so long. Feel free to get in touch with me if you'd like to share more! Hang in there! God Bless!