Youngest son & DP Don't get on :(

[crackalacking]

Have been with my DP For a few years and lived together for 10 months now .I have 3 kids from a previous marriage that ended 4yrs ago. He has one daughter age 5 who visits every 2 weeks.
My youngest is 12 and has been acting funny with Dp for a few months now and it's putting a strain on things. Everything is a grunt or and I don't know answer. He back chats him and doesn't say hello or goodbye ...the simple things. He seems miserable alot of the time particularly when I'm at work .Dp tries so hard to engage with him and include him in things he is doing with his daughter ....he doesn't want to join in and just waits for me to return.
This is not how it should be . Doesn't feel like a family when he wants to separate himself from doing things as a family .
I have tried to talk to him and he seems ok for a few days and then back to the same .
As I said its a strain on our relationship and we want to be married in the future .this makes me so sad ..... How can i fix this ??

Welcome to teenagehood! It's tough but it's get better. Harder for step parents who live with them as they usually get the worse of it but don't have the unconditional live we have to make it more bearable.

I do think it is something to do with his changing hormones but it makes my partner really sad ....he just wants us all to get along and wonders of I should let him be a bit more of a disaplinarion. .... they have never really had to be told off much as they are his kids but Dp can't stand it if my son back chats either of us . He wonders if he might be depressed. I dont think he is. My other 2 kids get on well with him but have had their ups and downs .
I get on great with his daughter and she tells me she loves me which is nice. Is it worth me sitting my son down and talking to him about what's wrong ?