"Short answer?" Delicately!
Although you are not this person's manager, in an HR sense of the word, you are the leader of your particular team. (Whether or not terminology such as "scrum" is used. No matter what, if any, "management philosophy" is used ...) You act with delegated authority.
Therefore, after privately discussing your strategy with the manager and listening carefully to his/her instructions or suggestions, speak privately (but, directly) with the developer. Remind him or her that your expectation is that s/he will stick to a task until it is completed, but also that s/he is expected to ask for help, assistance, and guidance. That such requests should be directed to you, and that you will accept them. That you consider part of your job to be to remove any impediment that is standing in his/her way.
Without being threatening in any way, try to help this person understand that "the team needs the team," and that his/her appropriate participation in these matters is, indeed, required. As team leader, and within the scope of the team that you are leading, you do have the prerogative and the (delegated) responsibility to say such things.
Having first taken the time to discuss your plan with the manager, you will now have the support of that manager. "Yes, Jim or Jane, I am aware that bobo2000 was going to tell you this, because he first discussed it with me. He was and is effectively 'speaking for me,' and I am your manager."
Also: be ready "at the drop of a hat" to stop and listen to this person. Why might this person be shying away from difficult tasks? The comment that "it's not fun" (or, what have you) might be a smoke-screen. Can you draw-out this person's candid view on the situation? Programmers sometimes conceal their perceived weaknesses, and they might well perceive weakness where and when you don't. Try very hard to make this a private(!), and two-way, conversation.
Practice your very-best tact, diplomacy, powers of persuasion, and ... decisiveness. Yes, you have authority. Yes, you do. But what you really want to achieve is: (a) to better understand the true situation from this person's point-of-view, and (b) to persuade this person to want to change for the better.
One fellow once put it this way: "Offer him butter and honey on a slice of bread. When he accepts, take out your sword and use it to butter the bread. He will take and appreciate the honey, and he will not fail also to notice that you bear a sword."
At the end of the meeting, and after letting the person have the last word, shake hands, walk out of that private room, and leave everything that was said inside that room, "inside that room."