L.L.
I would get something like white butcher paper and do something on that and then hang it so that it can be seen through the window. A couple of reasons: a) it's something that requires no clean up, just removal, b) it something you can keep, c) if you are having a party to welcome him home you could have everyone there write a love note (welcome home note, thank you note) to him either on the front or the back so he can see how much his service is appreciated.