First off, congratulations on the offer. That is fantastic, and a nice compliment to your son!
Just because a school is ranked higher does not mean that program will get your son better. Often at the highest level, especially for a walk-on, coaches will move down the line quickly if a kid is not performing. There is a line of kids stretching across the world who would jump at a Power 5 conference. For that reason, Power 5 coaches can always find another guy if a players performance isn't up to par.
If the goal is to improve and get drafted, you can't do that from the bench. Players who play get noticed. Here's an example; Coastal Carolina isn't power 5. If you look at their track record of draft picks you'll see 44 drafted and 7 more signed professionally. They have 4 current MLB players off their 2010 team. Developing players is key. If your son isn't a draft prospect right now and he wants to become one, he has to find the program that will get him better. So, take a look at this particular program and their track record for sending guys to the draft. Do they have their better players going to elite summer leagues?
Is it just about the draft, or is it about the total experience? If you want to just get drafted there are enough guru's to work with out there now who will train you for 3 years, get the velo to 93-95 and you'll get a shot. If you want to use college as a means to improve your draft chances, AND benefit from the experience by growing as a student, an athlete, and a person, then you have to look for the right fit, not the right ranking. If he's not drafted out of high school, many things can change over the next 3 years before he is eligible again. The question I always ask is, which school will allow me to reach my potential as a student, an athlete and a person?
As far as the promise to play as a freshman. That can change, and it's a little hard to believe, as others are saying. You should be 100% that you are hearing the coach correctly. There is a difference between having a fair chance at earning playing time and being guaranteed playing time. I'd suggest having your son clear that up with the coach. You still have to earn that when you get there. Even if you are hearing the coach correctly, the best players play. If you are 0-55 in the fall, or you're walking the farm on the mound, you are not going to have that starting promise anymore. However, if your son is getting a significant scholarship, and that promise, then clearly the coaches think very highly of him. It is important to go where you're loved. Do you want to be the guy who tried to walk on to a Power 5 and ended up transferring?
Go where you are loved! If you're good enough to get drafted, they'll find you.