He came back to his home town to celebrate becoming D-Rank, and no longer being a burden on the adventurer they partner with. They already knew his GF since she was from the village and left to become an adventurer with him and big sis, though telling the family they were now an item was another part of going home. He even seemed somewhat upset that his mother sold the potions, probably since she got majorly ripped of by the merchant she sold them to.