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Most important thing is that there are about 1,000 things it could be other than you. It could be you but he could be laughing to himself about a joke he just heard or he could be thinking about something funny/good that happened to him earlier in the day.
You're going to want a stronger sign than that. Eye contact is a big plus.
The source of the behavior you describe would be very hard to pinpoint. He may find you to be eminently attractive, and react in this manner out of shyness, but, on the other hand, he may think you look laughably ridiculous and be hardly able to stifle an embarrassing guffaw. Again, he might be freshly beyond the cusp of a particlarly virulent gastric emission, and tenuously restraining his perverse delight at the thought of your heading into the wake of its odiferous remnant. There are many more possibilities still, but these should stimulate your imagination.