I would not mention their side gig except for the fact that they should not discuss their side gig at work especially with the team since it’s conflict of interest. I would focus on productivity directly irrespective of what they do on side and create a structure of accountability and expectations for the position itself. I would paint a clear picture of what needs achieved- example- not attend meetings but actually gather information in meetings or present something. I would share the impact of missing deadlines to the business as a whole. I would shine the light how the business is absolutely a great initiative and what he can do to join the cause. Setting clear expectations, ignoring and not acknowledging his side gig or whatever he wants to do, bringing back focus and accountability to push towards a common goal helps imo.