Could you do as I have done before; note the license plate, description, direction, etc. Then get off the road somewhere where you could safely and discretely pull out your cell phone if you have one and report that there is a possible DUI and give the information, stating that you saw very erratic driving.
Leave out the fact that he tried to cut you off on your bicycle; in fact leave out the fact that you are on a bike at all. Just state that he's driving erraticaly and behaving as if he is DUI.
Many locations would take a call like this seriously; the more information you can give the better.