Hi D.,
First of all, it is normal your son may feel a little left out. Let's face it, the relationship has changed. Alot. It is great that you are making special time for the two of you. Your son will need that. The age difference is too great. Your children will not have alot in common for many years and their needs are different too.
Be careful not to be guilted into overindulging your son. This is life, he will have to adjust. There are going to be changes. My daughter, now 30 is seven years older than my son, now 23 and she always resented him. Frankly as she became more and more jealous and her behavior became more angry it was almost a self fulfilling prophecy because she just wasn't pleasant to be around and I actually did enjoy the company of my son more.
That does not mean I loved my son more. We do not get some finite amount of love to share, but we give all of our love to each child. Our children are not all the same so we do not treat them the same.
I will say today my son and daughter are great friends and it is I, who occasionally feels left out. lol.
I know small babies are a novelty, they are adorable, stay little for such a short time and require lot's of physical care; but, if you continue to find special time to share your sons interests your relationship will continue to be close and as your daughter grows more responsive to your son so will his love for her.
J. L.