Getting the boys involved and excited is obviously an integral part of making your Boy Scouts fundraiser a success. Kids, particularly those of a very young age, don't have a fantastic grasp of money issues yet, and a lot of what is involved in a Boy Scouts fundraiser may seem boring or dull to them if it's not translated into terms they can understand. Simply telling them that Boy Scouts fundraisers will "help the troop" is probably not enough impetus to get the younger kids excited. Be specific about how they will personally benefit from the fundraising profits, and you'll be a lot more likely to have them get excited about a Boy Scouts fundraiser.
Choosing a Boy Scouts fundraiser that suits the group can also help, as can explaining to the kids what the products are about. The benefit of certain Boy Scouts fundraisers is that there's no need to explain - candy, for instance, is a no-brainer where kids are concerned. They totally understand why that's a good fundraiser for their troop. Things like flower bulbs or coffee, however, might take a bit more persuasion. If the kids understand why you have chosen the Boy Scouts fundraiser that you chose (i.e. "because mostly we'll be selling to people like your parents, and parents love coffee"), then the chances of getting everyone on board increase greatly.
Mostly though, it's important to get the idea across that Boy Scouts fundraisers are fun. The kids get out in the community, which is what they do best, and the results of doing so will affect them directly. If they understand the relationship between Boy Scouts fundraising and the things they want to have and do as a troop, they're bound to have fun with it and learn the value of money in the process.