So sorry that your Granda is deteriorating. You're probably right in your suspicion that hearing the news that there's no more active treatment to offer him, has resulted in him becoming sleepier and less able.
He certainly has done very well, given his original prognosis, he's clearly a strong, determined and brave man.
It's likely that he'll sleep for longer and longer periods and not really want to eat and then not want to drink. It'd be good to have a word with the hospice nurse about what the plan is about his medications that will keep him as comfortable as possible.
It's difficult to start a conversation about how much you love someone if you've never done that sort of thing with them before and as you say, you don't want to scare him.
Touch is often a way of communicating without the use of words. You cut his hair, so maybe you could do his shaving for him and give his hands a massage. As the end approaches, the circulation is quite poor and feet and hands get cold. A foot and hand massage is very comforting and if you chat quietly to him, he'll be aware of the nice feeling and know how much you love him.
I feel sad for you, losing your lovely Granda. If I can help in any way at all I'd be glad to.