S.J.
Hmmm... Tough one!
I think the idea of two gifts might be your best bet since you have already done the barbies for the girls.
I think for a commemorative gift, you could do something like a letter for him to open later in life. Something sharing with him how much he means to you, what you feel like having him share this special day with you, etc. We did something like this for our kids' first birthdays. We created a time capsule. People put meaningful words and items in a "capsule" I created. Each child can open it and read it later. Same idea here....
Or, get him a savings bond or a CD - something that he will really appreciate down the road.
And then, I would get him that toy he has been asking, no begging, for over the past few months. There has to be one, right?! Get it for him! Is it a video game? My son was begging for a Nintendo DS at around that age.
Good luck L.!