These kinds of problems are not uncommon when your Basenjis start to reach their "golden" years. You might want to try him on Dr. Dodd's liver cleansing diet.
http://www.canine-epilepsy.com/Liver%20cleansing%20diet.htmMany have used it and had great results regardless of what you find out the issue might be.
Good Suggestion Pat as I had an older B with liver issues in 2008 and that diet brought it down into normal range.