Question

a person standing for a long time gets tired when he does not appear to do any work​

Answer 1

When we move around, our limbs move in air. There are changes on pressure on the limbs.  Due to the movements and changes in pressure, there are movements of muscles inside the body. They get relaxed and tensed.  So the pain experienced during the tension is relaxed quickly. We do not feel too much pain continuously.

When we stand at one place, there is a normal reaction force exerted by the ground on the legs. The muscles in the leg take this force. They get compressed as required. The weight of some limbs falls on the other parts too.  If we stand still at the same point, this compression force causes a pain in the muscles due to prolonged compression.

So we feel pain. But we are actually not tired in the sense of exhaustion after doing some physical work.  When some part of the body experiences pain, there is a system inside the body to relax that part by compensating and balancing processes.  There is some work done this way.

Answer 2

Because the earth is constantly pulling him towards its centre and having a gravitational effect on him... henceforth by standing his body is actually working against gravity (his body is forcing itself to stand which is against gravity) and so he gets tired