Maybe get some fun decorations for the bike, or to combine this suggestion with pwdeegan's, host a bike parade
. Invite other kids to come over, with their wheels of choice, decorate bikes with balloons, streamers, etc, and then bike together to an ice cream parlor or coffee shop (for hot chocolate and pastries) or to a playground.
More durable decorations include baskets, streamers, those spoke bead things (eg http://www.amazon.com/Sunlite-Bicycl.../dp/B000AO9Q92
), lights, bells, horns, etc.
My guess is that you'll be more successful if you wait til spring. In the fall and winter I wouldn't blame him for preferring to be cocooned in the trailer, depending on exactly what kind of weather is going on. Any success you have now may be short-lived, as the weather gets worse.
Also, get him a nice bike. The specialized hotrocks are really light, and much more comfortable to ride than a clunkier bike. If you're lucky, you can get one very reasonably used.