You will always need insurance if you come here on an OA. Even when the visa expires your extensions will require insurance. You will need insurance for the second year and if you travel you will only be stamped in until the expiry of your insurance. If you retire here using a non-immigrant O visa then you will not need insurance.
I don’t think you can switch from a non-OA for the entire first year of the non-OA. It is a multiple entry visa the first year. You can’t kill it by simply leaving without a re-entry permit.
The insurance question is very complicated. There is no way to recommend to anyone else other than to point out that it is necessary for the OA visa so you should think about what you are going to do when you need to extend or go back for another visa.
If you exit and reenter before this first year expires then you can get stamped in for another year and I think you need to renew that US insurance. What you need to think about is if you extend this visa inside Thailand then you may need to buy Thai insurance or look into getting your US insurance policy to state that they meet the Thai requirements. The point is the insurance valid to get the visa is not necessarily the insurance required for a 1 year extension. Getting a non-O visa decouples the your health insurance from the visa and you may have more health insurance choices. I came on an OA visa in 2017. I purchased Thai health insurance when I realized that my US policy wouldn’t adequately cover me here living full time in Thailand. My Thai insurance qualifies to extend my OA each year.
If the answer is No (his choice) then he needs to pursue a non-O. Otherwise he can continue health insurance with either a non-O or OA. But you know this . . .