We moved home when my DS was about 2, and DD 6 months. We only moved 5 miles but DS was desperately unhappy at first and would ran after visitors cars as they left because he wanted to to go back to the old house.
A local church ran a mother and toddler group and I went along to it. It was a godsend to all three of us. DS has always been a sociable type and once he made new friends in our new location he settled down. We all made friends, several of whom remain friends nearly forty years later.
Perhaps, if it possible with one child with health problems, join something similar, nowadays, grandparents are as welcome at these groups as parents. If he makes new friends at the new location he may begin to settle, plus the other suggestions already made.