Putting dog to sleep - how to tell fiance?

So my family (parents and sister) and I had a long talk and we've decided that in a couple weeks, we are putting my dog to sleep. He is 12 years old, has had surgery on both back legs, and over weight. He can barely get up on his own and sometimes when we try to help, he snaps at us. In our eyes, he's suffering and has not been the same dog I've loved all his life. We want to put him to sleep before it gets worse or he injures himself. Well my fiance is a Marine and deployed right now and loves him like his own. I warned him before he left that that could be the last time he saw him. We'll I haven't talked to him in the last 3 days and probably won't for the next 2 weeks, after my dog is already gone, because of lack of Wi-Fi and field training. How and when should I tell him? Next time I speak with him or wait till he comes home in February? UPDATE *** I'm going to talk with my parents and see if we can change it so I can talk with my fiance first. The only problem is we leave the 23rd to visit my sister for christmas in Texas. We wanted to do this early enough that we could enjoy our visit but still all be together during this hard time. Nothing can make this situation better but nothing is more important than my Bucky's comfort.