Firstly, I think you are probably wrong to consider a reply to an email taking >24 hrs is rude. Email was never conceived as an instant messaging tool – there are other technologies more suited to this.
Secondly, Did you agree how you would communicate or the service level you would expect either before or during COVID?
Thirdly, are you happy for him to Bill you £20 for replying to your email with “sorry no further information to provide at this time”?
Fourthly, have you tried emailing a generic office email or voicemail saying you are concerned you haven’t heard from him? It’s possible your message is in spam, or he’s sick.
Finally, have you considered whether your email was effectively written in the first place to elicit a response. I get lots of long rambling emails which mostly seem to be rants rather than asking something. I routinely ignore them. I’m clear with everyone I work with that if the purpose of their email is not clear without me scrolling it will not get read.