M.R.
Start this summer but make it more of a fun vacation. The visits don't need to be too intense but they will be great because he will get a chance to visit and tour campuses to start figuring out what he wants and likes in his next chapter of his life. The good thing is that prospective student traffic in the summer isn't as intense as it is in the fall. The bad thing is that he won't really get to see the 'campus in action' since traditional students will be off for the summer (but at this age, that's okay).
Your main goal should be to expose him to many different types of campuses: large, medium, small, public, private, urban, suburban, rural, and with different climates/cultures (liberal, conservative, artistic, etc.). This will get him started thinking about what type of atmosphere he wants in his collegiate experience. It will also help narrow things down for when he starts making the 'serious' visits the summer before his senior year and in the fall of his senior year.
Good luck, make it fun, and at this stage be open-minded. It's all about exploration.
And...remember - college is a match to be made not a prize to be won!