Step son self harming again

My step son started cutting again. His mom was supposed to come see him for his birthday, but didn't show up.. Again... This is the 4th birthday she has missed. This is like the 30th time she's cancelled on him and a majority of the time how she cancels is just not showing up. He turned 16 and told her he doesn't want anything, he just wants to spend the day with her. She said okay. Ignored all his calls that day but was posting on Facebook and didn't show up. He decided after that he didn't want to do anything for his 16th and just stayed in his room. He self harmed with the razer from his pencil sharpener. My husband made him go to therapy earlier this week than he usually goes. I feel really bad and don't know how to help. He got back from therapy an hour ago and my husband pretty much ransacked his room for stuff he can harm himself with. Idk if anyone has any experience dealing with someone who is depressed but can offer me some advice. I hate that his mom keeps breaking his heart.