You really really must tell your son the truth. Tell him in advance that Granny will be coming and as shiny said, talk about and plan what they'll do together so that he has a positive focus rather than just missing the nanny.
You're fooling yourself if you think it will be easier on him if you tell him she'll be coming soon. Easier on you, yes, but not on him. Children of that age are able to understand HUGE amounts. A child who keeps asking and is told 'x is coming soon' and then x doesn't ever come, does not just 'forget', they stop asking because they sense and realise that the reply they're getting is not honest. They kind of give up but it's a confusion rather than a 'it doesn't matter' thing.
You really can deal with this in a positive way for him. Telling him she'll be coming soon would be the worst thing you could do.