First and foremost, I want to add my congratulations to your son and his family! There are so many successful baseball players represented on this site, both college and pro, that some might at times almost forget what a huge honor and privilege it is to be offered the opportunity to play pro ball. This is the dream of thousands of boys who are associated with this site, and hundreds of thousands of players throughout the country, and your son has the opportunity to make this wonderful decision! Enjoy it!
http://www.hsbaseballweb.com/inside_the_numbers.htm
http://www.hsbaseballweb.com/probability.htm
Secondly, you've surely noticed that opinions differ as to what your son should do, or what type of financial offer might make sense for him to skip college and go directly to pro ball. Many of the members posting their opinions have recent direct experience with this decision. I think bbscout was mentioned - he is an authentic pro scout whose son was drafted this year after his Junior year of college but has chosen to stay and finish his Senior year at UCLA. TigerPawMom's son was a high draft prospect last year out of high school, but he and his family made it clear that he was going to college, and he has had a tremendous freshman year at Clemson, with his team still in the running for the College World Series. Others have spoken from experience about the advantages of going to college first. Another well-respected member who hasn't posted on this thread is OnePlayer'sPop. His son was drafted in the 20th round out of HS a few years ago and had almost decided to stick with his college scholarship offer, when he made a decision to follow his heart and accept the pro offer. If you check around on this site, you'll find some stories about that player, Zach Duke.
http://hsbaseballweb.com/eve/forums/a/tpc/f/6616002781/m/6781088801
All of these players obviously made good decisions, decisions that were a good fit for them.
I noticed that in your first post you mentioned about your son, "...but he really wants to go ahead and take his shot at the bigs." Many will tell you that statistically, it's very improbable that he or most other drafted players will make it to the big leagues. They are speaking from experience and a depth of knowledge, and I applaud them for sharing their advice. But the other side of the story is, the odds weren't very good for your son to make it this far either, were they? Whatever decision your son and your family makes together, there will be some who agree and some who don't. Have fun with this wonderful problem, and very best wishes for success!
