Visiting during the summer won’t really give you what you are looking for, so I suggest that he start working on schedule another visit in the autumn for whichever he thinks he likes best (before ED deadlines obviously). Sitting in on a class, being around campus, is really useful for this. It is striking how fast many/most students know whether a place ‘clicks’ or not, but it is harder when school is not in session.
If he doesn’t have a stand out ‘this is my place / these are my people’ school, I strongly discourage him from applying ED. If the thinking is ‘he’ll getting into a fancier name going ED’- hand on heart, it is not worth landing in the wrong place to get a fancier name (and it happens every year).
The unknown variable here is your son, who is going to change a lot over the next 18 months, so trying to guess now what will suit him even next spring- never mind the following spring- is risky. True story: by spring of Junior year, collegekid1 had a carefully curated, GC & parent approved, college application list, including an ED school (had visited, spent the night & sat in on classes) . By October there was an entirely new list, with only one school (not the ED school!) carrying over.
Leaving room for growth and change beats a tiny differential in perceived prestige.