Once boys start HS there are so many other things competing for their time that few make substantial progress towards Eagle after that (unless they have a "no license until your Eagle" millstone hung around their neck).

3 years is plenty of time to get Eagle, and it does teach a lot of great skills and provide lots of great experiences. And the deacons will still be on board with BSA stuff as far as campouts and summer camps go, so you won't have to do it all for your boy without some support in knocking out noghts camping, summer merit badges, etc.