He needs to talk to a lawyer from whatever state the conviction was in about getting the felony pardoned. In OK, pardoned felons can have their privilege to bear arms reinstated through a pardon if it was a non-violent felony.
Even muzzle loaders are prohibited for ownership by felons in OK, the way the OK statutes define "firearm."
If a person completes a deferred sentence for a non-violent felony, what is the status of their firearm ownership rights?