I wore one as a young child. Mom would decorate the sticky patch or let me pick a sticker to put on it. She put the sticky patch over my eye, then my glasses on top of that.
At this age (I have an almost 3 yr old boy also), I know how difficult it might be to persuade him to wear it. What I would try if it were my son would be to have two or three activities every day that will distract him for those two hours, yet also will let his left eye focus far and close. IE: watch one half-hour program on tv, then do play-doh with him, then he could help me cook or include a meal during that two-hour time. Perhaps he can have a small reward if he keeps it on for the whole two hours.