In my opinion CTC refers to the reccurring annual costs including actuarial costs of employment incurred by an employer towards an employee.
Retention bonus being one of the methods to ensure the continuous service of an employee to the organization, it is not necessary for an employer to adopt it as a regular feature of employment. It can also be ensured by means of a bond to serve a minimum period in the organization or initial engagement on fixed term basis to be followed by regular absorption. If it is a deliberate attempt to enhance your compensation package under a prospective employer, well, there is no guarantee that he would consider the retention bonus paid by the previous employer as an integral part your employability.
Therefore, my personal view is that the inclusion of the one time retention bonus into his CTC by the poster may not have a tangible impact in salary negotiation elsewhere as he expects.