Join the Union ASAP and have a union rep come with you to be the said witness.
Fill the union rep in about all that's going on and what has been said to you so far, when you rock up to the meeting just say this is my witness for the meeting today but dont tell the boss who exactly he is, once he starts being a cunt and saying unreasonable things the union rep will pipe up and inform him who exactly his is and probably tear him to shreds for all the bullshit and harassing statements he's made, you'll probably see a dramatic turn around from the boss as you have a reliable witness who can back up your claims of harassment and can even make certain jobs on particular work sites unpleasant for the company if they want.
Nothing to lose really by getting into the Union.
That's my advice anyway as I seen it work many a time before.
