If you cull him and want to have a necropsy done, put him into a plastic bag and into the fridge.This will keep the tissues preserved for testing. Do not freeze! This will destroy sensitive tissues.
There is no cure for Marek's and culling is the only option. It's a heartbreaking disease
If you do cull him, call your local extension office and find a lab that will do a necropsy. Most states have labs that will do them for little or no cost. That way, you will know for sure what...