You can put subtasks under stories, and Tim can have his own subtask that represents his contribution. You can also (with an extension) put time against subtasks if leadership/management want to get really pedantic. Each team member should have at least 1 assigned story/task/bug in each sprint, and Tim should have at least x hours logged against subtasks off stories in the sprint.

Or you can, you know, trust your team to deliver the stories they said they could do in a time window, as a whole. Let them figure out the how at the team level.