Tufts might be a good fit for your son.
It has a relatively small engineering school embedded in a small research university with an arts and science focus.
Bucknell’s and Lafayette’s engineering departments are about the same size and Rice’s is over 50% larger. This makes the ratio of engineers to liberal arts students at Tufts very low for a university and actually lower than some LACs. Since engineers are not housed in separate dorms, there is lots of intermingling.
IR is one of the most popular majors, so the population of IR majors is slightly larger than the population of engineers. If you add in the political science majors, the total population of policy majors is 50% larger than the population of engineers.
This makes for a large population of people who like to debate policy.
In fact, Tufts recently beat Yale for the national debating championship.
http://tuftsdaily.com/news/2016/04/21/tufts-debaters-win-national-championship/