He works as software engineer ---> This is wrong. You need an article. He works as a software engineer ---> This is correct. I'm not sure why you don't want to know which sentence is correct. If you already know that the first sentence is wrong, I'm not sure why you are interested in the meaning of it. 'He works as software engineer' is a grammatically incorrect sentence, and you shouldn't use it. End of story. As for 'meaning'... well, they both 'mean' the same thing. They both convey the same information. We would understand you if you said or wrote the first one, but we would notice the mistake and know that you weren't a native speaker. If you wrote or said this in an exam, you would probably lose marks for it. If you wrote or said this in a job interview for which you needed fluent English, it might lessen your chances of getting the job.

Further to my answer [above], and having seen what Su.Ki. has said, I want to point out that, although it may be grammatically incorrect, [and assuming that he is the only such employee], you could hear someone say : 'He works as software engineer at Keemun Motor Parts', for example, rather than the correct form : 'He works as the software engineer at Keemun Motor Parts'. Of course, when answering the question "What is his occupation ?" or "What is he doing now ?", you would say that he works as a software engineer.

If there is more than one software engineer in the company, then you may say 'He works as a software engineer . .' If, however, he is the only one [software engineer in the company], then you can say 'He works as software engineer . .' This is the only difference in meaning, and only in the sense that it implies a difference in context.