You're correct that it's absurd; services to build your Legos for you do not exist to my knowledge.
I would try taking an evening and organizing the parts that you have. Make it a parent-son thing - order a pizza, put on a movie, spread out everything you have on the floor and start separating the parts into groups. My suggestion: First group by general type (plates, bricks, hinges, etc), then by color.
Then comes the fun part: rebuilding. If you remember any of the sets that you have, you can download building instructions from the Lego website here. Once you're done separating by color/type it should be trivial to find the parts you need for any particular set.
Or just eschew this entire process altogether and encourage him to build something entirely from his imagination. Either way, your encouragement and/or involvement is a good thing.