It seems difficult to pin down Bossche's predictions as right or wrong.

Has his previous theory's been proven right?

His previous claim, as I understand it, is that mass vaccination will produce immune evading and more dangerous variants. I think the fact that the variants we've seen so far either emerged before widespread vaccination (alpha), or in places with low vaccination rates (the rest) is at least weak evidence against his claim.

Perhaps he would argue the low vaccination rates did produce those variants, even though the rates were low. But how could we tell?

Will his current theory's been proven right?

He says:

Omicron is likely to start out as a mild disease because short-lived, poorly functional anti-S antibodies (Abs) that resulted from previous asymptomatic infection (e.g., with another previously dominant variant) will no longer recognize Omicron. [...] However, the overall pattern of ‘mild’ disease would only prevail until Omicron becomes dominant and causes high infection rates. When this happens, short-lived, low affinity anti-S Abs will start to compete with innate Abs in an increasing part of the population as a direct result of the enhanced likelihood of re- exposure shortly after previous infection.

How would we distinguish this from the case where Omicron seems mild at the beginning because 90%+ of covid cases are mild, and then once the numbers are high enough we start seeing the smaller fraction of severe cases?

There is one prediction that looks almost tractable enough to (eventually) decide:

It is undeniable that mass vaccination will only drive the virus [...] to use alternate receptor domains on permissive cells. The fitness cost that may come with such a dramatic mutation is likely to be rewarded with enhanced pathogenicity. I am truly afraid that these dynamics will eventually allow for the natural selection of individuals with uncompromised innate immunity while eliminating those without it. While such natural selection would lead to an eradication of SARS-CoV-2 as innate immunity sterilizes the virus and blocks transmission [...] the price paid for ending the pandemic by virus eradication is not comparable to the one paid for by generating herd immunity and allowing the virus to enter an endemic state.

So I guess if we reach endemic state following mass vaccination (the general concensus is that eradication is basically impossible by now so I judge this extremely likely), then his theory will be proven wrong. And if many hundreds of millions die followed by eradication of the virus then he was right. Check back in a year or two?

The problem is that he can still claim half-credit if the virus uses an alternate receptor domain, but that doesn't lead to a high kill rate. And regardless we still won't know whether his counterfactual herd-immunity via natural immunity route would have avoided this.