There are several ways to meet the financial requirement:
Category A: Employment Income (with employer more than 6 months)
He has been earning £18,600 salary with the same company for at least 6 months and will continue to earn this in the future (either in the UK, or moving from the US to a new branch of the same company in the UK).
Category B: Employment Income (with employer less than 6 months)
He hasn't been earning £18,600 for the last 6 months/hasn't been with his employer for 6 months yet, but HAS earned that amount over the last 12 months from all income (either in the UK or the US). He also currently has a new job paying £18,600 (either already in the UK), or has a job offer in the UK that will be starting within 3 months of moving back (if still in the US)
Category C: Non-Employment Income
Between you, you have an annual income of £18,600 or more from non-employment, which will continue in the UK... for example, income from property you rent out, interest from stocks/shares, child maintenance, university grant etc.
Category D: Cash Savings
If he has no UK income, you need £62,500 in cash savings, held in your bank account(s) for a minimum of 6 months.
If he does have UK income, but it's not £18,600, you can make it up using savings - the first £16,000 don't count, then you need 2.5 times the difference between his UK salary and £18,600 on top of that.
Category E: Pension Income
If you have pension income of £18,600 or more, you can use this
Category F and G: Self-employment Income
He must have earned £18,600 average in self-employment income over the last financial year (F) or last 2 financial years (G), and can prove it will continue in the UK
See the financial requirement document for more info:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/337420/Annex_FM_1_7_Financial_Requirement.pdf