Mounting behavior by puppies is not sexually based it is an indication of high excitement. Think about the world from your pup's perspective. In the last 24 hours everything he knew and that gave him comfort disappeared completely. He is a sponge for learning but there is too much to easily take in. You are working super hard to help him settle, but it is all pretty overwhelming. Humping burns up a lot of energy and that is really all he is trying to do, get rid of some energy. Either ignore it so it is not rewarding to do or redirect him to playing with a toy or doing a little bit of bonding type activity.