Need help 😦

Hey guys , I need your help , so in the last few months my ex and I have tried to be friends it was going so well up until 2 months ago when he got with his new girlfriend since he's been with her he's changed so much he's always moody and annoyed and he has gotten a huge tempor on him , he won't even try and let me help him when he's in a mood like he used to let me. He used to be always happy and when he did get in a mood I'd be able to talk to him and get him out of it but now when I try he won't let me because "she might get upset". His little sister texts me everyday and asks if he's okay , recently we got into a huge fight because I wouldn't let him worry about me , I'm not his problem he doesn't need too. But it was so big we didn't talk for 3 days. I honestly feel like I'm loosing my best friend and have no idea what to do. I love him so much he's such a great person but when he's with her he's completely different and not him. What can I do to get our friendship back and not fight as much?