I would suggest getting a O-gauge layout software program. Designing your own various layout plans is (now) a major portion of the enjoyment of model railroading. When you have designed about 5 or so layouts that are possible for your space, analyze the pro's and con's of the layouts. Pick one out and build the support system for the track.
Get enough track down to run a train as soon as possible. Build from there, making changes that seem right as you work with what you have down. Add scenery, and make track changes that may arise, if any do. Have fun. Nothing has to be permanent. Your son is growing and his likes and dislikes will change as he matures. Allow for changes.
