Is it the older one? My DH is like this for our oldest and I wonder if it has something to do with becoming a first time parent with the first. I don't understand it -- it comes across as favoritism. He does everything objectively equally - buys the same for both. But if the older one is hurt/upset/disappointed DH reacts completely differently.
This happened over spring break: Younger wanted to go get a game from game stop. DH says, yeah, sure, one of these days on the break. Younger asks 3 days in a row, DH finally snaps at him to stop asking, we'll go when we go. Perfectly reasonable response to the pestering, although it was DH's vagueness that kept younger asking and asking and getting annoying about it. Older doesn't care so much about games, but he's into basketball shoes. I mentioned that I wanted to get them shoes on Sunday. I forgot it was Easter and it turns out the store I wanted to go was closed. Other stores were open, but this particular store is a little cheaper and those shoes are expensive so I wanted to at least check there first. I decide to wait until the following weekend.
Older son is disappointed and DH is saying to me, let's try other stores, why are you so tied to this store that is closed, etc. He's trying to change my mind so older won't be disappointed but he wasn't at all phased when younger was disappointed about the game; in fact he was irritated. This is a small example of how he treats them differently without realizing it.