First, WELCOME
!
Give it some time, and I think he'll heal. What I might do is, while petting/massaging him inch my way up toward his neck and massage/pet/comfort that area until he gets more comfortable. See how he reacts when you're massaging that area, and keep trying. You want to have him associate that area of his body w/ positive things/actions again. You may want to create an association btwn his collar and treats too - that may help.