J.B.
Does your school district have a special education parent group? My town has a very active group called the SEPAC that has monthly meetings and a very active Facebook page. The members are all parents of children with special needs or SPED teachers, therapists, counselors etc. who want to connect with each other. Members are always posting info for forums, support groups, workshops and other resources in our area. If I were you, I would check with whoever handles SPED at your school (might be a school psychologist, counselor or other administrator) and ask if there is a parent group in your area and if there is, you can probably get good info on local resources. Another place to ask might be your pediatrician or whoever is treating your son.
There is a group called Sibling Support Project that offers "Sibshops" in many areas. You can search their website to see if there is one near you.
https://www.siblingsupport.org/about-sibshops/find-a-sibs...
FWIW I think that looking for sibling support is a wonderful idea and hope that you find something near you!