Hi K.,
My first suggestion to you would be to have a long heart to heart with your daycare provider. Is your child in another person's home? Exactly what is happening in that home? Does your child behave in the same way at daycare as at your home? What does the babysitter do about it? Could your child be acting out at night what is happening to him during the day? Who provides the food he eats during the day, the provider or you? How much sugar does the child get over the course of the day? How much fruit juice, juice drink, soda pop, cookies? Is your son given anything in a bottle other than milk or water?
My first suggestion to you would be to make a hard and fast rule, nothing for that child but milk or water and see what happens.
18 months old often does like to throw things. He may be a born "ball player" meaning that he just likes to have things to throw. I have a friend who sets up a hoop for her ball player before he was even walking. It may be that this child needs as much outside time as possible. If he can't get outside to run around while at home with you, because you have the new baby, then you have to make sure that your daycare gives him as much playtime as possible. If he is sitting in front of a TV, or taking vey long naps, or expected to be in a playpen all day long, then he may be expressing his energy in throwing the moment he gets to the safety of his own home.
Really your 18 month old baby is still a baby, and because you have the other little one in your arms, it's easy to expect him to be more older and mature than he really is. If you ever used a baby sling or baby carrier, you might want to consider giving him some very close Mommy or Daddy time that way, especially once your little one goes down for a nap. Sometimes that very intense closeness can make a big difference. You might find that even taking a long bath, just you and the 18 month old, no interruptions, no other children, alone in the warm water will help to calm him down and give him the intense loving attention that he craves. I had to do that with my son once or twice after his baby sister came, and it really helped.
Just some thoughts,
C. Hill