I'm with Steve on this.
Basicly, because a weirdboy MUST use his powers, he is not allowed to go to ground.
Being pinned isn't an optional action, and prevents people from doing things. So if he's 'gone to ground' because he's pinned, he can't use his powers.
To make a similar reference: if a WH priest is included in a unit, this unit MUST assault in the assault phase, this means that if it's in assault range during the shooting phase, they may not fire rapid fire weapons because this would mean they cannot assault, however, if this unit is pinned, they can't assault because they are pinned, it would be non-sensical for them to be able to suddenly assault while being pinned, while they are not allowed to, however, if a unit would be within 6" of them when shot at, they would not be able to go to ground, because this would preclude them from assaulting in the following turn.