I think youre a good person for not wanting to hurt his feelings. Having said that, I also think you have to tell him. Try correcting him, like "you mean library?" And when he answers, ask him why does he write like that. You can be casual about it, and then, whatever his answer is ("because is cute", "because it saves time"), just say how you disagree ("nah, it barely saves time and it can be confusing. I prefer full words" or whatever). I think it's honest and not harsh, if he gets defensive then he is indeed a child, and if keeps doing it, well.... adiós!