Awww -- Happy Birthday, Little one!
I know it must be bittersweet. I would echo the idea of marking the occasion now even if it's just buying a little present to keep for him, then also celebrating once you bring him home. Maybe the best way to get through it mentally is to tell yourself, this is his last birthday WITHOUT you and that from now on you will be marking all these special days together.
Hang in there, and I'm pulling for quick travel news so you can be with your little guy. Best wishes and God bless.