In my opinion, you should be honest with the UK people IF indeed you confirm that he was previously denied by Canada. However when the time comes for renewing his US visa, he needs to maintain the old story he told them. Switching a narrative to immigration midstream can have disastrous consequences.
Practically you want there to be consistency in your stories to different countries immigration databases because you do not want to trip up over lies. As it is said, the truth is one but lies metamorphose and keeping track of lies becomes problematic. At the same time immigration and consular officers and immigration law can be very rigid and unforgiving and people have been punished for coming clean on hitherto undisclosed transgressions.
We also need to straighten out some information. Yes the USA and UK and other Five Eyes countries share information with regards to immigration. They however are not real time sharing every detail about every applicant for an immigration benefit among themselves. I know because I know several people who have conflicting information with different Five Eyes embassies and successfully get visas. Even within the same USA immigration system I know people with different answers to the same question on forms. Americans themselves will tell you if it ain't broke, don't fix it. Note I am not advocating you deliberately set out to give false information.