I just recently went through a similar situation. The only difference is we didn't live with my mom. Since the day my dd was born she has not went more than 2-3 days without seeing my mom. Even if it were just for 30 minutes or so. But every weekend my mom would come and get her at least for a few hours, and then beg to keep her all night. Some times she did and some she did not. So when we moved we went through horrible seperation pains. Unfortunatly we moved almost 11 hours away. One thing my mom did was she put together pictures of her with my dd and her with my dd uncle and just random pictures of our close friends and family. Then she put then in this little frame that displays one at a time, or you can take it out and look through the pictures like a picture album. My mom decorated the frame and everything. I have to say this alone probably saved my sanity, my dd had just turned 2 when we moved, so she didn't really know what was going on, except that when we moved she wanted her meme. We also talked about her all the time, and called her everyday if not more. anytime she wants to talk to her meme we call. We also go back for very frequent trips and my mom comes here. So that has helped as well. I think the big thing will be include him in every decesion you can. When we moved we made a BIG deal about my dd having a big girl room and a big girl bed, and every night before bed we wouold look at the book and say our prayers.
Basically all I can say is hang in there, It will get better, and also reassure your parents that he will not forget them. It was as hard on my mom as my dd because of this.
keep us posted.