While Sophocles didn't come out with the truth, he didn't tell a lie in its place, either, so I wouldn't exactly calling that lying. Concealing the truth, yes, but not lying. And it's not as though he never tried to tell them the truth; he did. He even acknowledged that the day of the farewell party was his last chance to tell his friends the truth.
When he learned he had misunderstood about his parents moving and that they were only moving down the street, he was genuinely afraid of what his friends' reactions would be if he told them. He thought they would outright hate him for getting them all emotional "over nothing." This is a kid who clearly has some insecurities about what his friends think of him, so I totally understand why he was so nervous to tell them the truth. (Remember, he wasn't even going to tell them he was moving at first. They essentially needled it out of him.)
I do agree that the end of the episode could have been a little better executed--it would have been more realistic if Ash and the others had been at least slightly annoyed--but I think the point here is was for Sophocles to learn/realize that his friends care more about him than he thought they did.
Also, I think it's more in character for Ash to be relieved that his friend isn't moving than annoyed that he went through all the trouble of getting a farewell present "for nothing."