At outstations like BOS, you stand a greater chance of them not weighing the dog, or weighing him and letting it slide. This is highly unpredictable, and solely depends on discretion of the staff on duty. With bag and bedding, he'll be about 2kg over the limit, that's borderline.
If they do weigh him and deny transport in cabin, yes, you can check him and he flies in the hold - space available and crate requirements met (you need to have one available right then).
The only 100% safe alternative is booking your flights on ITA (via FCO). That airline allows in-cabin pets with a combined weight of 10kg.