I don't have any kids but I am a teacher and while I am no expert, I think it is understandable your son would experience anxiety after what you all went through. His grandfather died and he probably feared you would as well.

I assume he is getting counselling? If not, that definitely seems worth looking into. I would also talk to his school if you haven't already done so. They may well have some supports they could offer. Schools are usually pretty understanding about things like this. I assume he has exams coming up? They could be adding to his anxiety.

I don't know whether it's PTSD or anxiety

Hi... Thank you for your reply.Yeah he's started counselling 3 weeks ago, I'm paying for it privately as the referral from the Gp to our local children and adolescent services is 12 weeks. To be honest school haven't been very supportive at all, he's in year 10 and when he started at senior school I spoke to the head of year and his firm teacher to inform them what he had been through, as while in primary school he spoke to a counsellor he dealt with things on his age level at the time. I informed his senior school just so they were aware of what had gone on, but because he's always been bubbly and happy I don't feel they are taking this seriously. I have spoken to his head of year and emailed her several times, she replied saying "he seems fine when he's with his friends!" I told her he's putting in a front in front of his friends as he doesn't want them to know how he's feeling. 
I'm just wondering if this is a form of PTSD as it just all seems to have come to a head now, maybe it's a mix of hormones, etc. He says school isn't the problem, he just doesn't know what's going on in his head anymore, feels he can't concentrate because his head feels full but he doesn't know what it's full of

This is just a possibility and is probably way off, but looking at your kids' ages, it looks like his little brother is now around the age he was when it all happened, so it occurred to me that that could be bringing back some memories, especially given what you said about his being anxious about his brother starting senior school. It might be reminding him of his experience of his last year of primary and moving into senior school. Not sure that is much help though.

Is there anybody else in the school you could talk to? I know a few teachers with that impression - that appearing fine means really fine. 

Hi, yea I hadn't thought about that in relation to his younger brother, that could have been a trigger. I will try and speak to someone from the school welfare's team. It's just so heartbreaking to my son who is normally so happy, sociable and full of life being so sad and anxious. I'm hoping the counsellor that I have got him seeing will help unlock and get rid of these harboured feelings. He has clicked straight away with his counsellor so that can only be a good thing.

thank you x