☆.A.
I'd see what the kids would like to get him--ask them!
Personally, I don't think you should buy him a personal birthday gift from you. Keep it about the kids for now.....
There's a lot to be said about being "the bigger person" but there's also something to be said for not trying to take personal responsibility for HIS birthday "happiness", right.
Part of a separation is a trial run to see what life is like without someone.
Good luck!