My Belizean fiancé was denied entry to the USA about three years ago, when he was attempting to go visit his daughter in Florida. He had travelled a number of times in the US with no problems. But on his last attempt at entry they suspected he was going to work illegally, and sent him back to Belize. There were no charges laid. At this point he will have to apply for a new passport, and is wondering how he finds out if he’s even allowed to land in the United States en route to Canada with me. How does one find out?
The US does not do "sterile" or "airside" transit: every person who flies from overseas and lands at a US airport must pass through US Immigration, enter the US, then pass through US customs. Only then, after being admitted to the US, can the traveler get to the departure gate for a connecting flight.
Belize is not a visa waiver country for the US. Thus, unless your fiancé has a US visa and a valid passport, he won't be allowed entry to the US. In practice, he will not even be permitted to board a US-bound flight. The only way to find out for sure if your fiancé can obtain a visa to enter the US is for him to apply for and be issued a US visa.
(Note that having a US visa only gets him to the US border at the US airport; US Immigration might still deny him entry. See Augustine of Hippo's Answer to this question.)
Without knowing more about the circumstances of his refusal — as asked about in @jcaron's comment to your question — no one can guess the probabilities of success.
Finally: this answer does not address COVID-related issues. Entry to the US, or to the specific state where the US airport is located, may require testing (perhaps multiple) and local quarantine upon arrival.