Is he in gcse year or the one before - i wonder whether the school would arrange for him to catch up at home, as a lot of the work is done online now he should be able to submit assignments etc and get homework on line -and revision help as well - if it is exam year maybe he could be persuaded to go in and sit them - or if its the year before then there is time yet, he could sit them later.
I think he probably is feeling protective of you, after such a trauma and also being an instinct to be the protector as the oldest son, which i know sounds old fashioned, but at 7 years old my grandson decided was his role after hs grandad died and still has that same instinct around me.
But also this is such an already pressured time for him without having to deal with what has happened on top of it - its a lot to ask that he carries on, and perhaps a good time to let him see that his feelings of grief an shock and pain are normal and sometimes it is acceptable to not be expected to just carry on.
While as adults we can make strategys for returning to work, and cope much better with our emotions, for a 15 year old lad the risk of crying in front of his mates or breaking down in the classroom is a far bigger and on top of that probably far bigger in his mind than it needs to be bur thats his age -and the fear of something being said that triggers that reaction is going to be much greater.
As adults we want to avoid it happening but know if it does it will be forgotten , people will try and understand,some will sympathise but theyre not going to call you names as you laeve the office at the end of the day When you are fifteen that is more likely and more massive.
I would be inclined to try to relax about what he is missing in the classroom this is a huge thing that has happened on top of all the other stresses about passing exams etc etc that children have now and i think that pressure cooker needs some time to release not forgetting they stay in education longer now there is time and the results may be better than those obtained under the kind of pressure he is putting on himself right now - if he is doing the work at home thats good, if he isnt then maybe you could strike a deal wtih him that you wont pressure him to go back to school until he feels more ready to do that, or maybe et a target of September if he has that extra year, but only if he agrees to work from home, and if needs be sit the exams later.
Maybe its something you could tackle together and in doing so he will talk to you more and open up about things and also see that you are going to be ok if he leaves the house for a while.