Yes, medical insurance is absolutely necessary if you visit the US.
Technically he will be allowed into the country, but without medical insurance, in the US (the most expensive country in the world to be sick in) any kind of accident or illness can result in him being liable for hundreds of thousands of dollars - maybe more. Provincial coverage of out-of-country care exists but is limited, and usually much less than US treatments will cost. Nobody should try to travel to the US without medical insurance.
Buying travel insurance for an older person can be extremely expensive (especially for travel to the US). Your auditor is very justified in not doing so. Most employees have medical insurance that includes coverage while abroad, but your self-employed auditor probably does not.
If you really want your auditor to visit the US on business for you, then it is normal for you to pay the auditor's expenses (plane tickets, hotels etc.). Your best bet is to consider medical insurance to be part of those expenses, and recompense him. Since it's an expense he would not be incurring if he didn't visit you, that's very reasonable.