With the violin or fiddle under your chin, it is very hard to sing whereas on your chest it is not acoustically coupled to your bones and it easier to sing with it.
Most of the fiddlers here were classicaly trained; but then again the schools don't offer anything else.
Laura, I suspect that some 8 year olds who don't want to learn classical violin might be put off by being forced to do it. I guess it depends on the personality of your son.
Whatever you do, I hope he enjoys it.