I am not a lawyer
I think carrying concealed inside the business (unless he is somehow part owner) would be against Michigan law (750.227)
I think carrying openly is the more interesting question. Michigan law doesn't allow anyone to sell to a minor but I couldn't find anything regarding possession. The GCA seems to have an exception that allows a firearm and ammunition to be "temporarily" transferred to a juvenile for employment purposes:
(2) It shall be unlawful for any person who is a
juvenile to knowingly possess--
(A) a handgun; or
(B) ammunition that is suitable for use only in
a handgun.
(3) This subsection does not apply to--
(A) a temporary transfer of a handgun or
ammunition to a juvenile or to the possession or use
of a handgun or ammunition by a juvenile if the
handgun and ammunition are possessed and used by
the juvenile-
(i) in the course of employment, in the
course of ranching or farming related to
activities at the residence of the juvenile (or on
property used for ranching or farming at which
the juvenile, with the permission of the property
owner or lessee, is performing activities related
to the operation of the farm or ranch), target
practice, hunting, or a course of instruction in
the safe and lawful use of a handgun;
So again,
I am not a lawyer, nor do I ever intend to be one. Just throwing this out for discussion purposes.
Ryan