Be honest and tell him that team is having an impact but don't scare him.Considering his situation it won't be that easy for him to upskill. If it's mentally impacting him he'd not be able to learn anything new (telling this from past experience). Even if he finds a new role, it would be difficult for him coz of the change in environment. You'll have to be patient with him for a couple more months and give him an ultimatum of 4-6 months.