How to be firm and direct?

So this guy that I have been with for about 5 months wants me to give him a blowjob. We've had sex so comfortability seeing each others parts isn't a problem. I want to give him a blow job, but I don't like the way he asks for it. He basically demands it and it makes me uncomfortable. I feel like I'm not being straight forward enough or being direct enough. I've told him before how he says it turns me off. I want to do it, but I feel like he's not taking what I say seriously and respecting me. Any advice or do I just sound ridiculous?