There is no problem with the designation as long as you have marked his role against the hierarchy defined for the organization. If you have defined a "Consultant" designation you have to map it to, for example, an "Asst. Manager cadre" or a "Manager" cadre. This would be important to have clarity on into which category he falls for certain benefits as an employee.
Also you have to clearly identify his role and responsibilities. Often it is difficult to define such roles when compared to more accepted designations like Manager, Customer Support Executive, Design Engineer, etc.
You could also consider, if it suits your organization, giving him a designation based on the organizational hierarchy( for example, Manager - production support) for the HR records and have the visiting cards to suit the business needs as "Consultant". For all career discussions, the designation from HR records holds good.
Hope this helps.