OK. I was able to read up on the topic of the American Defense Medal Clasps. The only one allowed by the Coast Guard was the one for "Sea". The Navy allowed for various other clasps. Since his rate both in the Navy and Coast Guard were aviation based, he may not have been on sea duty during that time in the Navy, but qualifed for one of the other clasps(Foreign Duty). When he transferred to the Coast Guard, the only clasp that would have been allowed to carry over would be a sea service(Vessel) clasp, so he may have lost the right to wear it when he switched services.
Also, I remember seeing somewhere that other service Good Conduct Medals were not allowed to be worn on uniform unless they were from the same service. I will have to see when this rule was changed. This may explain for the missing Navy Good Conduct Medal.