I believe that this is discussed in SA or MB explicitly. I further believe that what your son said would qualify as counting. I think that “Tonight is x” or “Tonight is y+1” are considered equivalent.
This reminds me of a story (though I’m not sure if this belongs in this thread, the jokes thread, or the pranks thread). On the second night of Pesach during Maariv, one fellow walks over to his friend and asks, “Can you be moitzie me with the bracha for sefira? I missed a night.” His friends asks with surprise, “How could you have missed a night? Tonight is the first night!” At which the first fellow beams and replies, “Now I will be moitzie YOU with the bracha!”