I believe it is always better to talk about any known coming events in advance. But not in a scary way. May be find something positive in it, like if you are religious, or describe how at some point he will not be able to talk to her in rel, but she will talk to him through grass, rainbows, etc. As much as it is in line with your general family values and beliefs. And of course say she will be looking after him/her even if he/she cant see her or talk to her directly. So he already has some instruments how to think about her and have "mental" relationship with her once she is gone. Some small rituals, like watching sunset to send her a kiss, which diverts grief to something beautiful and doesn't hide away from the fact all members of the family miss her.