Politics is all about pragmatism, dealing with what's happening now, and what's going to happen in the future.
That includes engaging with the current individual(s) with his/her/their hands on the reins, there is no mileage in engaging with previous incumbents who no longer have any power or relevance.
Politics is also brutal, and Boris will have no qualms about ditching President Trump in his 'lame duck' phase, anything he says now is so much white noise, Boris has to get on with engaging with the new man at the top, and he will be doing so right now.
Anything else would simply be peculiar - no-one would imagine that President-Elect Biden will be lining up meaningful exchanges with Teresa May - why on earth would he?
The here and now is what matters, and President Trump is becoming a disappearing part of it, whether hw wishes to acknowledge that simple fact, or not.