We don’t know anything about your son’s stats, but most of the colleges mentioned above are very tough to get into. Union College could be a good choice for a student with not stellar stats.
If Mathematics and hard Sciences (such as chem and physics) as a major are in the cards, many LACs are going to do a great job with that. If it’s strictly engineering, LACs with good engineering are already mentioned above. It’s getting in that’s the hard part. Agree with the suggestions in #3 above.