P.M.
If you had no choice because you are going to work, fine, you have to do what works. But, since you are home, and you are thinking you need him to go to preschool for 'him'. I say forget it. Why spend the money or the time and the stress when there is not a need? He doesn't need to be taken away from you and the baby so he can feel right, he may not even feel right then. Maybe he will feel you are sending him away and keeping the baby. If this is for you, I understand, but if you are trying to sacrifice and do this for him, don't. He doesn't need it. It is good for children to see us put babies first. It is good for them to have to wait a bit and see that mommy is caring for the baby...now he knows how you took such great care of him too. I don't think your son is being neglected at all. I think he is learning what it means to be a brother, to have a baby in the house, and this is not something you should feel bad about. Maybe when you are with the baby you can also be with him. I found when I made a nursing corner and everytime I fed a baby I read another child a book it really helped...especially helped keep the older child from seeking mischief. Haha. Whenever the baby would be asleep, even for just ten minutes I would play with the older child(ren). My children probably missed being the center of the world when the new baby came, but that is a good thing. Teaches patience, teaches wait your turn, teaches babies come first always...because they do.
Just include him in all the things you do. Try not to refer to him as "the big boy now" so he doesn't feel replaced, and then just enjoy the two little ones. Thank him for snuggling with you and try not to separate him form you or the new baby. Soon the little one will be older and the two year old will be older, and this will all be much easier. I think I survive each new baby by wearing them...keeps me with free hands for the older ones. And ditto to the post that suggested finding a nice play group to go to with them so he can play with other children with his momma near. Kids really do not need preschool...but I bet lots of us mommies are sure we do. Haha.
By the way...one of my twin girls is an Olivia...nice name.