When adding another vehicle to the policy you would also add the driver of the vehicle to the policy. Generally, insurance companies will not allow a person not living in the home to be on the same policy. If that person operates your vehicle frequently, talk to your insurance company. You may be able to add them to your policy as a non-resident driver. If you are allowing someone to borrow your car for a few days, he or she has coverage in most cases under a standard plan. Since you would be adding the driver and the vehicle it is unlikely your insurer will allow this. He would need to get his own policy.
If he is away at school, and he is taking his vehicle to school it is generally acceptable to keep/add the student on your insurance. This is acceptable because their permanent residence is likely still your home.