I think Confused is missing some information in his answer. It would depend on the contract if the designer is being contracted in the first place by the company and not employed or even an intern. If this individual is on contract he might be putting his initials on the work to help aid the director or lead designer in knowing what he is producing and it might be edited later in the design release. This would be up to the company and the individual you mention. Have you asked why he is doing it?
If you are an employee I would advise checking to see if you signed a non-compete or asked for a release on showing the work you produce. Some companies allow it but require that you state it was done while working at X company. Some companies might just simply tell you no. I think your question has base but just simply saying he is putting his initials on the work doesn't really state if he is using the said work in a portfolio or if its used outside the company.
If you have no documentation on if its ok I would advise on looking at the ethics within the company and seeing how this individual is handled. If the company comes back and says no dont and you proceed in some areas that is deemed terms for termination. If you are worried I would just simply ask because its better than assuming but provide a means of validation.