First, you should inform your Project Manager and share your doubts and findings. Then, the PM is organizing an 1x1 meeting with the developer to have a better understanding of the situation and discover the reason for underperformance. If the situation is critical, also a dedicated person from HR can be engaged by the PM. In most cases a conversation is the first step that helps; then we can all agree on a ‘repair plan’ with performance goals and deadlines. If it won’t help we’re engaging your Delivery Manager and will start looking for other solutions, e.g. new developer to join your team.