How to prepare child for parents going on holiday alone

My son is asd and highly anxious with seperstion anxiety too . 

Myself and my husband are off on a once on a life time honeymoon for 10 nights in a couple of weeks , my son will be at his grandparents .

I haven't mentioned it yet as I know the worrying he will do from now until we're back again , but I feel I also need to prepare him . 

Tips and advice on how to do this would be great , in away that won't cause lots of distress . 

 I've already cried myself a million times at the guilt of leaving him.  

Thanks in advance