I think he needs to bond with you first, then gain his confidence back. Wait till you see his reaction around other dogs before making that decision. Could be that his submissive ways would be taken advantage of by other dogs and make him even more withdrawn. You will know in a few months if he should have a little buddy or if he would be better as an only dog.
If in a few months you do decide to get him a brother or sister, let him decide. Let them be together for awhile first. If he seems to be submissive to another dog, then I wouldn't get one. The other dog would always domineer him which could lead to problems.