Do you plan on getting married? I am just curious, judging by your age and life circumstances I am assuming this isnt an option. My mom is divorced from my stepfather, has been for years and is 66. Even when she had a long term BF she said she would never get married again so I get it.
I do not think this is ok and if you plan on staying together I think you should insist it changes or move on. Your love is someone you want to spend special occasions together with. If his mom actually hates you and decided to not let you participate then he could go to see her the day before or after thanksgiving and before or after christmas saving the holidays for you. Even if it means you are primarily spending time with each other and your family, it is his mom that has decided that things change not you. To plan extended vacations away from you is weird and hurtful but you have to set your expectations and boundaries about it and then hold him to it or it will never change. At this point let the focus of who changed the arrangements fall by the wayside. It doesnt matter if it was his mom or anyone else what matters is that he is abiding by it and cutting you out.