Who is his deputy PM if he can no longer fulfill his duties while in ICU?
Dominic Raab, First Secretary of State
The Foreign Minister Dominic Raab is standing in for now. If Johnson dies, or is forced to resign it is a bit unclear how a successor would be named on a more permanent basis. The PM is the leader of his/her party in the Commons. Normally there would be a party election while the outgoing PM remained on until a successor was confirmed. My guess is that in a worst case scenario, the Queen would formally ask Mr. Raab (with the consent of the cabinet) to serve as acting PM for the duration of the emergency.