Wow, a page and no-one has yet done an anti... so I will.
I dont think he can use any psychic powers if pinned. Though this is hard to argue by rules; I would say that 'pinned' is a universal rule that doesnt allow ANYTHING to be done while under its effects, that the codex doesnt override it because it is not saying that teh weirdboy 'always' rolls at the start of the turn, simply that he must- In my mind the difference is that a 'must' is an action that you have to carry it out, if you are unable to do so you ignore it (see below).
I think that my position is basically decided by my being a space wolf- they have another rule which is a 'must' but which is often impossible and which no-one would try to argue that you can try to do anyway:
Head-Strong: Blood claws must charge if one of their models is within 6" of the enemy, unless the pack is being lead by an Independent Character...
It being a "must" means I'm unable to shoot RF weapons (to carry out an action that stops me from being able to carry out the action), but I can run into
6" of the enemy (or disembark a moving rhino within 6") and obviously I can not charge- I am not fleet.
The rule gives the "only" exception, and yet there are others that are results of BRB rules which the codex doesnt overwrite.
While using other codeces to try to argue rules is fround on, this is not what Im trying to do- simply to show precident on the wording. If 'must' means that you yourself cannot do anything to stop being able to carry it out then I think the argument is more "can a weirdboy choose to go to ground?" (I would say no- codex doesnt override BRB, it simply means you cant use the relevent rule).
However, getting pinned is not optional and I would say that you would have to skip the weirdboy roll (it leads to non-sensical results... can you do x, y and z while pinned, do you get the power but cant use it, does shooting/DS while pinned end the pinning, ???).