To be honest I think he should build the tour based around locations he wants to see. I was recently down that way attending a wedding and the riding is fantastic. I would highly recommend that he rides the crater rim of Mt. Aso (120km) then plan his rides around similar scenic features.
There are some amazing cycle paths as well - these are incredible and go for miles and miles, some even have bike hire and repair services at either end.
Oh and thee interior has a lot to offer as well - coastal roads are fine, but imagine Enoshima on a weekend.