Just explain to him that having a doula will mean that he can actually enjoy the moment, come along for the ride, rather than having the responsibility all the time to do things you need him to do.
As I have mentioned before, I personally would not really ask him. If I'm the one who has to do the work, then I do it my way. By all means, explain to him why you would like her there. It is much easier if he is on board with the idea when the time comes. But do not give away your power of birthing the way you want to birth. Yes, he is the father, but let's face it, what would he know about what YOU might need when you're in labour. Hey, you don't even know what you will need until you're in the situation.
I thought I might need massages, hot packs, people helping me to be in various positions. But when I was actually in labour, the only thing I needed was someone offering me a drink every now and then and to leave me alone most of the time.

I hope I don't come across patronising. I don't mean to be. I just want to let you know that it is alright to insist on these things. If you back down on this and things end up not going to plan, you might hold resentment and blame his resistance to a doula for the rest of your life.

Sasa
xx