he would get 15 years max of NI years if we backfilled from 2015/16 and continued paying until we wanted to start claiming benefits.
As Durhamlad says, if it's all voluntary contributions it won't be affected by WEP. So the comparison you'd want to make is between the pension this would entitle you to, and investing the money elsewhere.
Class 3 contributions are currently around £880/year, so 15 year of NICs would come to £13,200. This would give him 15/35ths of the state pension, or £4,002 per year.
That's a pretty good deal. As a guaranteed index-lined income and ease of use, it's probably hard to beat. But let's guestimate what compound returns in the stock market would do on that initial £13,200 in the estimated 10 years between now and when he would claim the state pension.
Presuming your husband is still subject to US taxes, most of the UK tax advantaged accounts will not be available to him, so we're looking (based on US historical data) at an average 4% after-inflation return, which comes out at £19,539. If he does have access to tax advantaged accounts, we can use a post-tax return of 7%, and the number increases to £25,966.
Presuming he plans to live for 30 years after retirement, he should be able to withdraw 4% a year with a 95% chance of not exhausting the capital (and a 50% chance of ending with more than he started with). Which would give him an income of £782 / year (£1,039 / year if he was able to use tax advantaged accounts).
It's not even close. In your place, I would absolutely make those NIC payments. The only scenario you loose out in is the morbid (sorry!) one where he dies before or shortly after making pension age.
I personally have a suspicion that the state pension could become somewhat means tested before folk like us get to retirement age. However, if you're rich enough for this to be a risk, you probably aren't dependent on the pension anyway and can afford to roll the dice. If you're poor, investing outside a pension also has disadvantages when applying for care home fees, court judgement relief, scammers, etc.