I don't have kids - yet, one's on the way (that I know of, heh heh). I don't know your kid and how he'll react to news that he's going to WDW months away. Thinking long-term, I'd gently suggest telling him so far in advance will be awesome in the moment, but it'd probably behoove you to wait until the date is closer, like within a week.
Reason the 1st.: some kids handle anticipation and expectation better than others. Having to wait months for that satisfaction will set up many expectations in his head, some of them may be unrealistic. And one thing kids do well, is imagine. If his dreams don't meet his reality - the heat is overwhelming, he's not constantly surrounded by characters - he misbehaves & you have to yell at him or even punish him - it won't be as good a memory. The less time he has to anticipate what will happen, the greater the odds are that is expectations will be met. We probably all know kids who had a meltdown on his/her birthday party. A lot of that comes from all that anticipation, and things not working out exactly as planned.
Reason the 2nd.: Even the best parents are guilty of the occasional emotional blackmail: From "Behave yourself or we're leaving the movie right now" to "I'm going to tell Santa to put you on the naughty list," everyone, sooner or later, uss threats to keep a kid in line. And if your children misbehaves between his bday and the trip, there might be an urge to say "That's it, we're not going to Mickey's house." It's either an empty threat & he knows it, or he doesn't know it, so when he convinces you to keep the trip on, he thinks he knows to manipulate you, which causes more problems later.
Sure, I'm probably overthinking things, or I'm just a pessimist. But were I in your shoes, I'd hold off until a few days to a week before telling him, max. You still get the look of surprise, you still get the joy of anticipation, but there's not enough time for it to REALLY sink in for him, so it all seems THAT much more awesome.