Semen can vary in appearance from person to person, and can also appear different with the same person from one day to the next. Whether there is sperm or not in your boyfriend's semen won't change the appearance. But, you may want to at least check with a fertility specialist about having a sperm count done--especially if you have not gotten pregnant after a year of unprotected sex. Good luck!
Is what you are looking at definitely ejaculate, and not just pre-come? Pre-come tends to be clear, but ejaculate should definitely be white/cream and cloudy. If this is ejaculate then yes, I'd say something is wrong here, and a doctor should definitely be consulted.
Having said that, I don't think that a lack of sperm in his semen would alone cause it to be clear. I had a vasectomy (after conclusively proving my fertility three times, which I felt was enough!), so there are no tadpoles in my semen any more, but it is still the same colour and consistency it always was.