I know it is easier to say than do. But please, treat your first born as a 'normal' child too. I know you must love him as much if not more than your baby just because of the problems he has, but still, if you think that he is not 'normal' and can not bring you as much happiness as your new baby, your actions will somehow reflect your feelings, and the child will be sensitive enough to pick it up.
He is just a little different, and hopefully not something he can not compensate by excelling in some other areas.
It might sound cheesy indeed, but I truely love the American way of saying that 'Everybody is different and everybody is special,' especially for little kids.