No way girl stand up for your rights!!!! if it would help you, you need it. This is on eof the toughest most rewarding things you can do and you need to grab as much support from whatever sources you feel the most comfortable with. Sell it to him!!!!! Tell him that it would mean if he is needing a break that you will have a much needed support person, what if he needs to step out for a breather, a loo stop, to chuck or pass out, food a good cry??????Some one who is experienced who will no what is happening is invaluable a will help keep you focused, partners are great but are often to emotionally involved when it comes down to the nitty gritty. Maybe you could suggest a trial and if it does'nt work out the doula will leave you and him to it, once it's happening he won't want her to leave!!!!!
Last but not least, you could always cry and beg him to let you have one....tears and preggy women work wonders lol.