How to we break it to the family?

This is not up to you to tell them. They are his family and he should be the one to tell them. I am sure they will get over it and if he explains that you would both feel it would be unfair to your family and that it is also your honeymoon then they should understand. They may be upset but they will have to get over it. As long as he sticks to his guns and takes no discussion over it then it should be resolved. can you not have a celebration when you get back so that both families can celebrate with you?