The DNR regional offices have the paperwork you need to get started. He goes to his doctor first ,then to a therapist for testing and evaluation. Get the application signed and mail it in. DNR reviews and approves it then sends it back to your father.(make copies and file the original) Mine took a couple of months so start now and you will have no problem with the season starting before you are ready. No matter what you hear or read, crossbows are not magic killing machines with 50 yard range and pinpoint accuracy but using one is much better than staying home. Good luck to your father.
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pj