It's hard to point you in the direction of specific resources since I don't know the specific field your child is interested in, but you still might consider organizing car washes, bake sales, auctions, etc., and approaching service groups and related professional organizations for sponsorships to help defray part of the cost, especially if your son pledges to do something for the community with the knowledge that he gains from going on the trip - maybe acting as a sort of local ambassador for the museum in question, making presentations about his trip and his subject of interest at meetings of the service or professional group and at local schools. People are much more likely to pony up money if they see that the benefit is not going to be limited to single individual.
