(1) The controller and the processor shall designate a data protection officer in any case where: ... (c) the core activities of the controller or the processor consist of processing on a large scale of special categories of data pursuant to Article 9 or ....

Article 9 describes personal data as data revealing racial or ethnic origin, political opinions, religious or philosophical beliefs, or trade union membership, ...

I would say that messages send via IM are personal data like described in Article 9. I also would check the "large scale" checkbox. So in my interpretation he will need a DPO.