There's not a lot you can do, since you actually don't have any levers to pull to make things go another way (from a hypothesis that you aren't a decision makes in your team). So, in addition to the previous answer, I'd recommend you to:
- Make each member's performance clear and obvious. Try using a task management system or bug tracker (Asana, Trello, Jira, dozens are available). This will help you show that a certain person is reluctant to take responsibility. Give this advice to your project manager.
- Since the system is set, stick to it. Avoid chaos. Be a good administrator of your work and let other members understand it and behave same way.
- Use tasks and subtasks. Make the very little aspect of work visible, even if it is about communication or planning.
- At the end of iteration, on deadline, output a report which shows what and how much each member has done.
This approach is pretty administrative and process oriented and doesn't involve people's preferences. This it may backfire. First, it takes much time, as you need not just do your work but comment on it, too. Moreover, such a change doesn't come easy. If you don't use a task management system then the change may be even painful.
But remember: the final goal isn't to push out a low-performer but to gain some data and knowledge about his productivity (among other members). At the end, it's not important how good or bad or nice he is, it's important to meet the estimation.