is this Parental Alienation

Well where to begin I am a grandmother of a beautiful 4 year old girl I am well versed in the seperated families both parents been married 3 and 4 times and myself and 3 of my siblings been divorced . I am lucky to have a wonderful husband now and have been married for 18 years. My sons marriage broke down 2 years ago lasting only 11 months .He has allowed his ex passports let her move on with her life and move away with her new partner and his daughter.having 50/50 parental resposibilities court orders and travelling a 6 hr round trip once a month with 4 weeks a year every thing was ok until 2 months ago when he informed her he had a  partner. She started sending horrible text messages and got doctors certificates with referrals to phsyologists saying his daughter had issues with his partner . he would ring her 3 times a week to talk to his child and now was told dad i dont want to talk to you you are not my family .his next visit was for a week starting boxing day he was told that she had another appointment and his solicitor was informed that he would not see his daughter for the christmas break.he tried to ring her on christmas day and was told she was asleep .he finally got on to her 2 days after xmas and she called him by his first name and said she didnt want to speak to him anymore .He is heartbroken how can a 4 year old change from a loving caring little person to not wanting to have anything to do with her dad Is this Parental Alienation and how can we stop this happening f he doesnt get to see her.