I was recently exposed to the chicken pox and potentially my dd. I talked to my ped. and she said it is unlikely for a baby that young (2mo.) at the time to catch the chicken pox. Most baby's are immune to catching the chicken pox until 6mo. She said if she did catch it she would most likely only have 1 or two pox marks and that is it. The baby would not have a full break out which means she could potentially catch it again later on in life.
As someone already mentioned you can't vaccinate babies for the chicken pox but they could have recommended it for your toddler.
My dd did not catch the chicken pox just like the ped. said
You should still keep the 3 year old away from the baby as much as possible and wash your hands everytime you touch your son and go to touch the baby.