I asked him the other night why is he not in the christmas mood? He said cause there are no kids. So I asked him Did your family ever had a christmas tree and decorated it? He said not really cause his parents were always busy and he never got what he wanted. I said No wonder you don't care about christmas! It's the thought that counts and not the actual item. My family always celebrated christmas and we had a tree every year. I always look forward to xmas cause it's the only time of the year to hangout with family. Then he said Oh sorry we can get a tree if you like? I said Nah it's ok. We can buy xmas stuff after christmas since it will be cheaper.
In conclusion I guess he didn't care cause it wasn't a tradition in his family to celebrate it. I will definitely change that next year.
If he needs kids to be in the Christmas mood, then he should be buying gifts for nieces and nephews - yours and his, or find a volunteer opportunity where they deliver presents to kids.
Also, find out what he didn't get as a kid (which made him such a sour puss), i.e. what he really loved, and get it for him (from ebay). Maybe Transformers, Voltron, GI Joe, She-Ra, Jem, whatever....