After you get married can your fiance stay in the US for the full 90 days on a visa waiver or does he have to leave immediately?

You can file a I-130 petition for him, and in theory he should be able to stay. The one issue is that he got married while he was travelling under the VWP. An alien using the VWP must NOT have the intention of staying in the U.S. permanently. So the issue is one of INTENT. If you already had plans to marry before he came into the U.S.- then he has commited Immigration fraud, and since YOU must have known that you were getting married, you filing the I-130 would also be looked on as fraud. In the past, if you got married toward the end of the 90 days, the government wouldn't really question intent and would approve the I-130. So if you are in the above situation, and if the I-130 is approved, know that legally (i.e. what you did was against the law) you (and husband) had no right to such approval and your case somehow slipped through. Now, if he travelled under the VWP and while here, you both had the SPONTANEOUS ( meaning no previous thoughts) decision to marry, then this would be a bona fide VWP entry as there was no intent to marry before travel.

However, there are recent reports of USCIS cracking down on adjustment of status (AOS) requests from immigrants travelling under the VWP. So can he stay? short answer is possibly, but although this was done in a very dirty fashion, youre fine as long as the immigration agent handling your case doesnt look too carefully at your file. If you got married within 30 days of entry, then i would say youre pretty much in trouble and to expect a long and detailed interview with USCIS. Remember that if the USCIS agent denies the AOS your husband would have ABSOLUTELY no appeal, the agent's decision is FINAL (one of the requirements of travelling inder the VWP is that the alien WAIVES all rights to appeal to the immigration courts.