Thank you for being here and deciding to join. I'm so sorry for the loss. It sounds like you really care for your grandfather and I'm glad that you are here.
Unfortunately there are no quick fixes for grief. Even if you did somehow have the capacity to be with him 24/7 there would still be intense moments of loneliness. So while being alone at times might be incredibly painful, it really is something that he has to do to cope. I do wish there was an easier way other than to 'feel the feelings' which does sound obvious but is obviously difficult to do.
smallwolf has given some really practical advice- is your grandfather open to socialising or getting any support (general or clinical)?
Perhaps there could be a way to shuffle around your time and his time- having routines is so important, so even if that means a call or a visit (no matter how small) from different members of your family as a way of staying connected.
I hope this helps a little