coach told him to "block" the base‏

[DG]

If the fielder has the ball he can block the base in any code. If he does not it is OBS under FED (and NCAA). Under OBR a play would have to be imminent to not be obstruction. I have seen in print (PBUC I believe) that the ball is judged to be inside the grass cutout at the plate (13') to be imminent so at 3b it would likely be the same. Unless there is some malicious action by the fielder no reason to eject. Malicious action by the runner could be also.

So, depending on rule set, make call as you see it. Can't see a coach getting ejected for coaching kid to obstruct. Just enforce the obstruction if you see it.