Would you feel this is obvious and unfair favoritism on grandmas part?

It sounds to me like she is playing favoritism. For your sanity I would try and distant yourself from that. My mother always played favoritism to my brother over me when we were younger. It always hurt my feelings. It was almost mental abuse to me. I distanted myself from her over time. After a few years she finally came around. We are actually really close now. I would suggest for your sanity to try and do this. You don't have to completely ignore her, but if you want to go somewhere else then go there instead. For example going to hawaii. If she wants to see your son make her make the first move to come and see him. you want a loving family around your son so if she can't provide that then it is her own fault not yours.