So you want a "Happy, but Sad " departure from this particular scene.
In my opinion, (again, it's just that and nothing else, I'm just putting this out there, and hopefully get some feedback that we can all work with here.) I think the 'sad' part won't be an issue; it's the getting his family "happy" about leaving. Which I think their happiness hinges on how realistic are the chances of him coming back to earth. So how REAL is that chance?
You've made it very clear that there is a REAL chance of him being gone forever. I don't/can't see a family walking away 'happy' with 'low to remote' odds presented to them. I see closure at best.
how about these angles to help shape the parting conversation;
1.) Maybe he knows for sure he won't be coming back, but told his family otherwise? (sad for him, happy for them )
2.) Maybe someone in the immediate family has a military background, who has deployed before. This would set the family up to being 'used' to this in a sense, may not result in them being happy about it, but they would have some sense of closure or understanding about it all.
3.) What setting is the departure scene taking place? Will it be in front of a transport ship? in the living room as he's leaving the house with his stuff? The location might play into the emotional value as well. "Home is where the heart is." Might be more emotional saying goodbye to a child as they leave the house they grew up in vs. watching them walk on a transport ship bound for space.
Anybody else see something that could help the dialogue or setting for his scene?