I think its a good learning opportunity for him. We can't protect our kids from disappointment all the time. So if you buy something it's just perpetuating the impression that life is fair...and it's not.

Maybe next time he mentions it, just casually say sometime like "I wonder too honey, sometimes people forget things like this, so try not to get too excited. If they replace it then great, but if they don't it just means they are busy. I'm sure they will get something nice for you next birthday." It's hard to see your kids disappointed, but that's reality, and it's no reflection on him or the people who have forgotten. It just is what it is. Instead use it as a chance to teach him something and make it not too much of a big deal.

Sorry you feel stuck in this difficult position xox