Just saw this and wanted to say there are many reasons for this but you should let hour OB know.
In your case it sounds as if this is new and that he previously nursed both sides equally. So like the others I'd think it is simply change in supply, increased sucking effiency with baby or side/position of neck or head preference when nursing.
If your supply seems to be decreasing you can increase your fluids, I found drinking more milk helped me. Oh and of course not all sides are equal, many women have one breast that produces more!
Not to scare you bc the Internet does enough of that. And again based on your description I'd think it preference of baby. I'm simply including this as mamapedia now has ability to search and this is a sidebar for others looking for answers with more sever refusal of one breast. Infection and or cancer can distort the milk in one breast and therefor baby will refuse.
Again does not sound like your case based on baby's age, teething and previous behavior. Either way I'd still mention to my OB to see what they say, of course consult peditician too for confirmation on age appropriateness on this new found preference.
Sounds like your going to be weaning at your baby's pace.
God bless,
mb