You do realise that your fiancé can only VISIT the USA and not live there without the correct visa? So, using VWP if he is a British citizen, this will be for 90 days?
Ah, yes - good point. If he goes straight back with you, he can only stay for 90 days, he can't work or legally live in the US and he would need to be able to prove that he is just a visitor, that he does not intend to live in the US and that he has a life (home and a job) to return to in the UK. If you are newly married and are starting a life together, it may look as if he is trying to use a visitor visa to live in the US illegally.
If he is going to live with you in the US, he will need to apply for a US spousal visa, which can take on average 8-10 months to be approved, before he can move to the US to be with you (first you will need to file a visa petition in the US to sponsor his visa which can take about 4 months to be approved, then he will need to attend a medical in London, and a visa interview, and then wait for the visa to be processed, which can take up to another few months).