I think what you're referring to as rainbow thread is what I call variegated. There are a number of different color combinations available. If you can't find any at LQS or JoAnn's, etc.
www.superiorthreads.com has a great selection.
All the different colors with your meandering would look great on the back. I'm not a very good quilter so I'd use black so my quilting wouldn't be so obvious on the back.
He'll love the quilt because of the memories and because you love him enough to make it for him. My brother even suggested to me that I offer to sell racing T-shirt or photo quilts for fellow racers because he loved his so much.