Lots of good points on this thread Marksmith. Keewart's point was the first thing that came to mind to me--he's a position player. Things (spots and recruiting timetable) are different. My son was similar to yours 5'9 1/2 " 145 as a senior in HS, SS/2B/CF. He was looking HA schools whose timetables are later as test scores/grades for JR are usually needed..though those timelines have moved up as well. His earliest offers were D3 --he was continuing pursuing D1 that were still on his radar, but weren't ready to pull the trigger. He would wait until early summer jr year, when ranked Big State U made him an offer on a visit, which then and one of his two dream schools offered that same night.
I also like PA baseball's suggestion about contacting schools where he is interested in and informing them of the offer and getting a sense on their timetable (indirectly asking where your son is "on the board').
Your son has to ask himself, what else is out there? Is it better? Which school has his major? How much does finances play into the decision? Is distance from home a factor? Roster size? If he is receiving this offer this early, it likely won't shut down the coach from recruiting other MI (certainly time). It's a hackneyed expression but true: Consider his school by looking at a 40 year not 4 year commitment. Your son is in an enviable spot. And if he didn't play baseball is the school with the offer at or near the top of his list. Good luck to your son! Definitely spend some time at the school with the offer.