”he’s off the computer” = ”he is off the computer”
What you want the meaning of is the phrasal verb ”to be off sth”.
What this phrasal verb means here is that you have been doing a task/work using that thing (in your example the computer), have completed that task/work and now physically leave that thing.
So, in your example, after he has finished working / doing sth on the computer, he leaves the computer then watches football on TV.
AussieInBg2 weeks agoPleasure! Razi72722 weeks agoThank you 🌹 vincent5 weeks agothanks