Support can be done from a distance, baby-sitting can't !! We support OH's children using a variety of communications, we have organised birthday lunches, small treats, surprises etc for them and managed it thousands of miles away. We even keep in contact with the schools on updates etc. We send child support on a monthly basis and still manage to put funds aside for them ready when they need the help with their driving lessons, first car etc.
You can do all of this with your son and new grandchild......photo's, scrapbooks, postcards are fab!
You mum is being unfair so try not to let her influence a change in your decision - as you said before your son is happy with your move and that's what counts.