Whichever breast you offered last, start from at the next feed. So if you offered the right breast, but he didn't take any, start from that breast at the next feed.
A breast is never truly 'empty', because whenever you squeeze it there will always be milk available. If your baby has had a good feed from that side (you will know whether he usually takes 5 or 15 mins to finish feeding on one side) and has stopped actively sucking, sit him up, burp him, try again on that side, and if he's still not interested give him the second side.