How many months before a passport expires can you still use it?

DH has just informed me, one week to the day before we leave for the US, that his passport expires in 5 months. I've googled and it would appear he's still OK to travel to the US on it, but I'm wondering if someone with more knowledge than me could just confirm that?

You do not say what nationality DH is. Guessing he is a Britcit and therefore comes under the Visa Waiver program.

For most countries the passport must be valid for at least six months from the date on which the holder enters the USA. If not, the holder will be admitted until the expiration date on the passport; check with the embassy.

For nationals included in the Visa Waiver Program passports must be valid for at least 90 days from date of entry.

All travellers entering the USA under the Visa Waiver Program require individual machine-readable passports. Children included on a parent's passport also now require their own machine-readable passport. Travellers not in possession of machine-readable passports will require a valid USA entry visa.

http://www.worldtravelguide.net/country/292/passport_visa/North-America/United-States-of-America.html

Check with the US Embassy though to make sure.

He's British. Yeah, I think we're good - had a pm chat with someone.... I already knew the bit about the passport and the waiver. I just wanted to clarify the expiry date. We're only going for two weeks, so even if they don't let him stay for six months it's all fine. Ta.

DH has just informed me, one week to the day before we leave for the US, that his passport expires in 5 months. I've googled and it would appear he's still OK to travel to the US on it, but I'm wondering if someone with more knowledge than me could just confirm that?

I think it's 3 months for the US as that is the period that the visa waiver is for, so it has to cover the maximum period of your visa.
On an associated note, and he probably has, but has he registered on the US Embassy site for visa waiver....this came into force earlier this year, you have to do it online at least 72 hours before you travel, it's then valid for 2 years.
I only point this out because we travel on Friday and I only remembered about it last week!

There is an exemption to the 6 months remaining validity rule for British Citizens, so your husband should be OK as long as it isn't set to expire while you're in the US, but be prepared that they may only stamp him in for the duration of your visit.

http://travel.state.gov/visa/temp/without/without_1990.html#mrprequirements

Passports, regardless of the type, must be valid for six months past the expected stay in the United States (unless country-specific agreements provide exemptions). This is a requirement for all categories of passports - regular, diplomatic, and official - when the traveler is seeking to enter the United States for business or tourist purposes, for a maximum of 90 days.