Struggling with a demotivated employee

Hi, I am looking for some guidance on dealing with a demotivated member in our team. By nature I am analytical and direct, which is possibly contributing to my employees underperformance.

The situation: healthcare, govt, operational duties. Employee's father works also in organisation in a senior role, but not in our department. Employee does not have a degree, and admitted being laid back, but at same time expressed on several occasions they want to grow into a leadership role. Yet, person is doing only half the workload of their peers, does not or reluctantly take advise on board, shows little initiative and complaints about my managing style to his dad, who subsequently gets union involved. Person does not complain to me or my managers. After their complaints, suddenly performance shoots up to expected levels, but drops after again.

What have we tried: increased levels of support, now with daily check in attempt to increase performance (successful for two weeks, then person slips back to reducing performance), working from office instead of WFH, on a daily basis give tips on workload, encourage them to ask questions (limited success) and also asking them to be more focused, by addressing in excessive mobile phone use in office. The person is very capable, but often not focussed.

I am covering managerial duties (not leading) as one of my team leads is on a 1y sabbatical. I have tried laissez faire management and directive management, both without success. I have been coaching person on getting to understand the bigger picture, but that results in them even doing less operational work afterwards. The big issue is that they want to grow but dont seem to grasp that taking ownership and meeting their peers performance is the way forward.

I am at a loss, and looking for tips, as all I and my peers do, seems to have effect for only one or two weeks at a time.