This is a bit complicated, and I can only guess as to what his reasons are. I know one thing, if you are reluctant to use your safe word for anyreason it will not be easy for you when you have to. Perhaps he is trying to get you to realize that the safe word is not a last resort, but a safety valve. As a Dom when I push a subs limits I want her to be able to grow, but I also want her to be comfortable with stopping me when necessary. If you are not comfortable with this, then how will he know when he is going to far?

I would not do it this way myself, but I do not think this is that unusual. A Dom and a sub must always feel free to communicate, and this may be his way of showing you that.