How do we get the caregiver to agree to additional help with Mom?

My Dad is the primary caregiver for my Mom. I go in 3 days a week to help with bathing, dressing, cleaning, laundry and meals. A sister also comes to help with cleaning.
On the days that we are not there Dad does everything. We think that someone else's help would ease his burden and give him a break. Sometimes he sounds all for it, other times he says it is his job. He gets impatient with my Mom when she wont eat or when he has to prompt her or feed her himself.
We just found out that she has lost 7 pounds in 3 weeks, probably due to a uti. We would like to pay for extra help in hopes that Mom will gain some weight back and give Dad a break.
Any thoughts? Thanks so much!