You probably want to ask this question on your son's specific instance of Moodle (school, college, whatever). This forum is about a thing called the "assignment module," which may be different from submitting an assignment (such as an essay or a report or a video). More generally, "Using Moodle" is a worldwide collection of Moodle users and developers discussing different aspects of this course management system.
But the short answer is to
- Go into your son's Moodle classroom
- Scroll down, looking at the middle column, until you see something like "Assignment #1."
- Click on it.
- Likely, there are directions right there, but even if not, there are two basic ways to submit an "assignment."
- Copy & Paste into a text box (like the one you composed your question in)
- Upload a file, which is typically done by clicking the "Browse" button, navigating to the file you have created, selecting it, clicking open, clicking "Upload this file," and clicking "Continue."