Well, after he reads a book he places it in a box or bag (to return to library or to your bookshelf) and you then record that on an ongoing list (books read the week of....). If he reads digital books have him update you once a week on what he has read.
I had a system....projects (or writing) were posted on refrigerator or bulletin board for a week or two then placed in a box. About 4 times a year we went through and kept favorite art, science, and history projects and writing samples for the portfolio. I made sure I had a beginning of the year writing sample to compare to the end of the year. The history program I used (story of the world) actually had you keep a notebook with everything so I had every history paper we did and was able to pick out a paragraph or worksheet from there.
If you are more unschooling you could have him write at the end of the week a paragraph or two summary of what he learned....like what topics did he explore on his own or with your help. When my son was young he dictated to me and I wrote it down. I would say today you learned about the Vikings...tell me what you learned...and he would tell me the big ideas and some details and I wrote it for him...later he took over typing that on the computer or writing it.
Oh. Also...use photography and keep pictures of things (we mummified a chicken during the study of Egypt). Store your pictures on a cd as a back up. Our portfolio also included pictures from scouts, homeschool PE group, home school field trips to plays and museums.
Our portfolio was a really cool academic scrapbook of our time homeschooling.
