I'm looking at the same issue now, except my LLC is only a NFA holding company.
^ What he said- although you might need to file amended articles to change to a Manager Managed LLC before appointing Manager(s).
Or you could make him (assuming otherwise qualified to possess NFA items) an authorized agent of the LLC, who (by your action) is immediately authorized to possess Company property. In your will, you will probably leave him 100% of the Membership Interests of the LLC.
OR
You could amend to make the LLC Manager Managed, appoint yourself as a Manager, appoint your son as a Manager, create a revocable trust now, transfer 100% of the membership interest to yourself as Trustee of the Revocable Trust, have a trust agreement prepared naming the Selttlor of the Trust(you) the Trustee (you) the successor Trustee(s) (Son, brother, etc) and the Benficiaries of the Trust (you, Son, others as you deem appropriate), and when you die the Membership Interest remains held in trust, or vests as directed by the Trust Agreement.
Your accountant/attorney might have some insight on the tax consequences of the devise at your death, versus setting up trust now, relating to the business purposes of the entity. Tthe NFA stuff would just be along for the ride.