Rengoku wasn't a drinker in the slightest because of his father being an alcoholic..

So, when you offered him to come to your mini house party that Tengen, Shinobu, Gyomei and Mitsuri would be attending, he politely declined when he heard alcohol would be involved, even though he really wanted to go, he had a massive crush on you. But, he lied and just said he had plans already.

As he sat in his bed, bored and lonely, he heard his phone ring. It was you! He got excited, quickly answering it.