I recommend that you make every possible effort to reduce stress for him now by not overstimulating him or overstructuring his life. Give him plenty of free, unstructured time to play. He will also benefit from lots of attention and cuddling, as well as some special, relaxed time with you every day, doing what he requests. I also suggest eliminating all TV and video viewing of any programs containing violent or frightening scenes. You do not mention your approach to discipline, but I recommend for all children gentle methods of discipline with no punishment of any kind.
Children know how to recover from past stressful experiences.
They do this partly through their play. You can encourage your son to re-enact through play past frightening experiences, such as doctor visits, saying goodbye to grandparents, or moving to a new home. For example, you can give him a doctor kit and suggest that he play doctor with you. He will probably find ways to use the materials and your attention to create meaningful, therapeutic play that will help him work through some of the frightening or painful experiences he has had with doctors and illnesses. Children also use tears and tantrums to heal themselves from stress and trauma.