It is entirely possible that something got lost in translation if English is not your first language, what exactly did he say? If his exact words were "I can write you a general LOR" then that means he'll write you one, but it won't be glowing and state that he strongly recommends you for graduate study. Instead he'll say general things like "Jess Q performed well in my classes," "Jess Q is interested in the field and intends to pursue a PhD at your school," or "Jess Q is suitable for graduate study." None of those say anything bad, but they aren't amazing either. Ideally you want your LORs written by someone who is going to rave about you, so if there is someone else who can do that then consider asking them instead.
