Should I tell my husband?

My husband has a son from his previous marriage. He's 16 and we get along pretty well since he's living with us and I honestly love him like my own child. I've known him for about 8 years now and today he came home telling me he has fought with a teacher and its pretty serious because he might fail the class.
His dad is working a lot and had a little operation a while ago and I dont want him to stress about his sons school. He's a smart kid and I'm sure he can do it and I'll talk to his teachers again but he asked me to keep it like as a secret from his dad. 
Should I tell him or not? I'm not sure to be honest.
His mother and I get along really good aswell. She was actually the one who introduced us (my hubby-her ex) and me and we're close friends but she works a lot aboard and is totally fine (has always been) with me being the 'mother role' in her sons life. I just dont know if I'm supposed to tell him as a wife or stand by our sons side as a good mother.