A: What makes you think the work is not his own? Be sure of your ground before you challenge him. The most common signs of plagiarism are: inconsistencies of style, untypical vocabulary and American spellings.
If you've noticed any of these, pick a phrase or sentence that feels different from his usual work and type it into the Google search box. This may reveal the source, in which case you have hard evidence. If it doesn't, make a big point, with the whole class, about signing the declaration on the coursework cover sheets - the part reading "I have produced the attached work without any help apart from that which I have stated on this sheet" - and outline the consequences of cheating. Give plenty of opportunity for confession.
If he still claims it is his own work, take him to one side and discuss the pieces that worry you. Check his understanding of vocabulary - if he can't explain the meaning of words he has used, the chances are he has lifted the text.
If you still have suspicions, tell him that you're going to ask your head of department's opinion, which should put him under a little more pressure.
Take the work to your HOD, with a written note explaining what you're suspicious of and why, and include any supporting evidence.
Better by far, of course, that the situation should not have arisen in the first place. Spell out, at the beginning of the course, your expectations about coursework, about the consequences of passing off someone else's work as your own, and insist on proper referencing and bibliographies.