Your story is very touching, Lu, and I hope my reaction will help in some way.
First, let me say that, if your grandson is in a good preschool, it has nothing to do with his disrespect and thoughtlessness. Helping children learn politeness and concern for others is an integral part of any good preschool curriculum, so I doubt that your grandson’s behavior stems from his experiences there. There are probably many other things that could influence his behavior but, as I don’t have all the information, I may simply be guessing. I’ll do the best I can by venturing two possible explanations.
My first guess is that he really likes preschool and is afraid that he will have to stop attending and return to being cared for by you full-time. If that happened, he would miss the contact with other children and access to a wider variety of play materials than most homes can provide. If he is concerned that this might happen, it should not be interpreted that he doesn’t love you. He may perceive you as a threat to that and think that, if he is ugly to you, you won’t want him.
My second guess is that you may have been doing too much to buy his love (bringing him presents, etc.). Parents who travel a lot usually learn rather quickly how easily that can happen. If every homecoming involves a present, all too soon the greeting stops being, “Hi, dad or mom” and instead becomes, “What did you bring me?” So I would recommend that you be very careful about bringing presents to him except on very special occasions.
Let me urge you not to let this break your heart. Don’t accept his ugly behavior (like sticking his tongue out at you), and tell your daughter to back you up. Refuse to care for him if he acts that way, and I predict you’ll get plenty of support from your daughter. Certainly it sounds as though she needs you for back-up care, if not for full-time care. Finally, get some interests of your own so your happiness won’t be so dependent on your grandson’s good behavior and manners.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.