He's right, Rhino/3dsMax/Revit/Ecotect e.t.c. are overkill for a simple gazebo and have a far sharper learning curve.
SketchUp's formidable, and learning it is pretty intuitive, though easier if you have someone tech-savvy around (even if they've never used the program before).
If you get around to it you could always upload images for help/feedback.
http://sketchup.google.com/download/
Scroll down to get the 'basic' version.
google seem to snap up half of the world's tools...
"Invest wisely and have money work hard for you"
Technically it's Trimble SketchUp now, the weblink remains to avoid confusion.
I may download that just for fun.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh
Show it to your son, he'd might find some use for it. Oh, and if anything, learn how to use the middle mouse button in combination with shift and/or ctrl to navigate.