Rail Trails and Greenways: suggest wide tires. Really. Clearance for wider than you think would be necessary.
The Vaya comes with 40mm tires, plenty of width for rail trials. I had no issues with 1.5's, 38mm, on the GAP and C&O during Hurricane Irene. The Vaya can take up to 50mm tires. He will have no issues with the Vaya in regards to tire width.