Thanks for all the replies and suggestions. As it is, both DH and I had a talk with the boys and just reiterated how their hygiene is perceived by others and that it's much more enjoyable to be in the company of someone who takes pride in their appearance and cleanliness (ie: smell) LOL
They were very receptive and were asking for ways that they could actually remind themselves to bathe/shower more regularly, and we assured them that if they stuck to a schedule of at least every other day, they would be ok. If they worked up a sweat, then they should take the initiative to jump in the shower that night. And since we go to the beach often, we always have them shower when we get home to get the sunscreen off, so sometimes this summer they were showering 2x a day.
Again, I think just pride in oneself is the key to be bringing to their attention and then letting them go from there (with the proper guidance from us).