"He contributed 7100 last year and is being taxed on the overage."

To one account, or to two? My understanding is that this would be split into each of two individual accounts, because they are not married and are not supposed to pay for the "other" individual out of your Taxpayer's HSA account. HSA is an individually-owned account; even if both are covered under the same high deductible insurance plan as registered domestic partners, that qualifies them each for having their own account, and the family total would be split across the two accounts.

Try this:

https://hsa.umb.com/employers/education-tools/faq/eligible+for+an+hsa/_domestic+partners+eligible