Boyfriends cum is clear?

So I have a boyfriend who has completely clear ejaculation.  See-through. Every time. 
Guys I've been with before have had thick whitish cum.  
I've said to him that I don't think it's normal but he denies anything is wrong and I'm wondering maybe if he is a blank-shooter/infertile and that's why it is so clear?  Can someone answer this?