Rent discount not specified in lease. (I want it now!)

I just moved this last August. I noticed after signing the lease that there was a 50% rent discount in section 7. No date or particular month specified. Thought it was a sweet deal as moving was really expensive.

On the 1st of this month I paid my rent, sending 50% of the total lawful rent. I emailed my landlord (he was on vacation last month) concerning the discount. I explained what I saw in the lease agreement since I didn't get a chance to talk to him earlier. Today I got an N4 form regarding the missing amount in rent, with 2 weeks until the termination date. I also got an email back, he said the "move-in incentive" applies on the 6th and 12th month of the tenancy. (I don't see how he can call it a move-in incentive too when it was never mentioned to me before hand and I had to dig through the lease for it myslef).

It wouldn't be a big deal for me and I'd just wait until those dates to use the discount. But my girlfriend is moving in with me in a couple months and will be splitting the rent with me anyway. Since there's nothing in the lease agreement stating when the discount applies would I have a case with the tenancy board? It would make my life a lot easier for it to apply now and it seems like their mistake for not being specific in our legal agreement.

All the information about the discount that the landlord gave