Stay still and let the tension pass. Pretty soon, he'll get over his concern, and if you're lucky he'll go back to doing what he was doing. Approaching him straight on, sneaking up from behind, or staring directly at him are all acts that he may associate with aggression, so be subtle. Blinds can be very helpful if you know where your subject is going to be, or if you expect to remain in one position for a long time. Marsh birds, for example, often live in exposed areas where you simply can't conceal yourself any other way.
Some photographers like to build blinds out of materials handy in the field.
But in our opinion this requires too much reliance on luck, as well as a whole bag of tools for assembly. It can also be disruptive to the environment. If you're going to do much of this kind of photography, it may be worthwhile to invest in a portable blind. Don't forget that you're setting up this blind because you plan to stay awhile. Comfort is important, so make certain the blind is big enough for you to sit without crouching a blind is rendered ineffective if you bump against the fabric and cause the whole thing to ripple.