A shadow box is the obvious first choice. You can display these in your home until your son settles down and wants them. I have a few of the open ones, like a frame only 1 inch deep. I also have a wooden one with a glass slide-in cover, and I put some small travel memorabilia in those sections (about 3.5x2.5 inches), backing them with card stock and some scrapbook decor showing where they are from: a little map of Italy for the Leaning Tower figure, some suitcase decals for something else, little passport embellishments, etc. You could buy some florist foam, cut it to size, glue it to the back, and cover with card stock or scrapbook page paper, then push the pins into it (super-glue them to stay once you get your design set).
Another option that was suggested to me - granted, this is bigger scale - was this: My husband has metal emblems from different European countries from when his parents traveled there many years ago. They are sort of like decals, but instead of sticking to the car, they were attached to the bumper sort of like a tiny license plate. An artist friend suggested I have them mounted on a car grill (like what covers the radiator) that could be picked up at a junkyard. She suggested I paint it and then attach the metal with wire, winding it around the grill sort of like a twist-tie. I'm not sure if you can attach your pins to something that way, but it's the kind of idea you could discuss with a craft store person. If you have a Michael's or a JoAnn Fabrics near you, they have pretty knowledgeable people.
