Maybe I can give you the perspective of a junior because I am one myself. The solution is just to sit down with him and discuss the problem. In this way he feels involved in the problem and you get his insights. He knows best whether he will succeed or not (if he has good self-knowledge).
There are three options that the junior can say. The first option is that he believes that the tasks can be achieved within the deadline. This is of course ideal and you have nothing to "fear". If you expect that he will not succeed within the time period, then it is a good idea to discuss the possible problems he may encounter. By doing this, he can do the job independently with your tips and the deadline can be met earlier. Do this, of course, rather than later.
The second option is that he says he doesn't know. Annoying and useless answer because you do not achieve anything with this. The best thing is to continue asking and trying to understand why he does not know this. This of course takes time and I do not know if you have this time. Maybe it becomes clear that he wants to complete the task but he does not expect that he will succeed. It is difficult, especially as a beginner, to estimate how long a task will take. Especially in R&D. If there is a time pressure behind it, an inexperienced person person will say more quickly that it will not finish in time and that the best option is that someone else will take over.
The last option is that he says he can't finish it in time. Annoying but hey, you can do the tasks by yourself or let somebody else do it. However, there is a catch. You gave him this task with the idea that he can finish it. I know from my own experience that I did not liked it when i was put off a project due a deadline. It feels a bit like failure because you have not been able to complete the task you were given. This is demotivating and my advice is to clarify why this happens. Do not try to get him completely removed from the task but have a part done.
If you suspect that this problem occur often and you want to avoid this problem, give the junior shorter but not necessarily less important tasks with a deadline or larger tasks that have less priority. This way you can keep him motivated, learn to deal with deadline without putting him of the project and in the end stronger.
I assumed from my answer that there is time to guide him.