Try Amazon's secondhand/'used' section when searching for books - I buy all mine using that, and so far, they've turned up and been in good nick. Not so sure how good it'll be for textbooks, but the fiction has always been fine... I don't think ds will need a computer of his own if there's one in the family home he can use when he needs to...or maybe I'm kidding myself - dd is 3.5 and am horrorstruck already about the fact she can use a mouse and load up the CBeebies website unassisted!
I'd wait and see what the college wants. I teach A levels in FE, and personally have banned my students from wordprocessing their essays - we found that they were typing so much that they were losing the ability to just sit down and write essays.
That said, it would depend on the subjects your son is doing and where he is studying. At my college, we regularly sell on second hand books from second years to first years, and I provide all my materials in the packs - so they need to buy the packs, and not any books.