C.N.
The first thing to do is definitely bring him to his pediatrician. Rule out any physical causes. As I'm sure you know, a urinary tract infection makes it difficult for even adults to maintain bladder control. The solution could be as simple as a few days of penicillin.
You said this started 3 weeks ag, and the exchange student has only been with you a week. Did this start while you were making arrangements for her arrival? Perhaps he felt that making plans for her was taking awya time that you usually spent with him, and became concerned that you didn't lov him as much anymore. Kids that age don't understand yet about love not diminishing when you add people.
Tell your doc about the exchange student. Your son could be concerned, and at his age, simply not know how to express it verbally.
If it is that he is upset about the new person living in his house, there are a couple fo things to try that might ease his mind.
Set aside a specific interval that is just his. Make it the same time every day, and write it in the calendar. Don't just work it in when you can,make it an unbreakable appointment. During that time, unless the house is on fire, he should have your undivided attention - you don't take phone calls, monitor laundry, or check your email during his time. It doesn't have to be a long time - at that age, their attention spans aren't that long - just something that he can count on every day that he knows will make you all his for a while. It can be as simple as the two of you sitting down together to watch "Sesame Street," letting him pick out three of his favorite stories for you to read.
You might also ask the student to make a similar appointment to let him know that she isn't taking his place in your life.