Wow aren't you lot a bunch of unsympathetic folks. There are a whole bunch of reasons for thinking this is unfair and rubbish and frustrating... just because in signing up to the armed forces you are agreeing to deployments (sudden or otherwise) should not mean you and your family cannot expect to be frustrated when it messes things up for them, or want to be supported when difficult things come up.
Lets be honest here too, this whole mess is the fault of a huge company, who promised the earth and could not deliver it and the armed forces are having to carry the can. Lots of friends family members are being draughted in at the drop of a hat and it had led to many akward situations for them, missing family holidays that were already booked, missing big family occasions planned for when they would be in country after deployment to afghanistan etc. with some families who can ill afford it loosing out on a lot of money, through cancelled holidays, most of which they will not get back and they are expected to just suck it up without comment or frustration? Military families have real lives too, we do our best to plan around these things, keep a stiff upper lip and all... but honestly cutting people a bit of slack and allowing them to feel the frustrations this throws up and work through them instead of the responses many on civvy street (as illustrated here) give of "well you knew what you signed up for...) etc doesn't help anyone does it.
OP In the grand scheme of things, No, don't worry. Your little one will not remember, nor even know the exact day of his birthday. If DH cannot bring him to you, then do something really special for him when you are back. I'm sure your DH will cope fine, is he serving too? It is frustrating that no-one has thought to ask if things are tricky for you with a last minute deployment like this, but don't let it eat you up. YANBU to be pissed off and frustrated at all. Wether your being in the military means you are more likely to be deployed suddenly. Someone else's crappy planning has led to this and I for one am seething on your behalf.
Hope it all works out OP. Do your best to make the best of a bad situation, and celebrate with DH and DS when you return. x