Get someone to walk at his head to begin with to reassure him. But have you considered long reining him instead; it might be easier for him until he gets more settled into only having sight in one eye. He probably feels very unbalanced on the blind side so lunging in circles is not helping him. When my horse lost her eye, she was very falling in on the blind side; if you are riding or longreining, you may find it beneficial to hold your rein slightly higher on the blind side. It balances them up. Worked really well with mine.
Why do you need to lunge him? I think lunging is really overdone. If you're starting a colt or if you need to use a surcingle to help gain collection then lunging is a good tool but otherwise I see no reason to do it. It's not great for your horse to go around and around in circles. It puts alot of pressure on thier legs since they are anatomically built to go straight.
Its just that its the only thing I can do with him at home because the field is too mucky to ride in at the minute and the lunge arena is the only place to work in at the minute and it is too small to ride properly in