Step dad has become weeeiiird

Hi,
First of I'd just like to say I'm typing on my phone now so please excuse my mistakes and all, especially that I'm French.

So,just before hand I'd just like to say my mum and stepdad have been dating since I was like 6 so since 2003, and I've just turned 18.
My stepdad had suddenly "shut down" when I was like 12, and he left my mum, and it was all very fast. According to some friends, he seemed very depressed after leaving my mum. They got back together after less than a year, got married when I was 15, had a baby when I was 16.

So everything was going on well up until, like, september, except from the fact that I cant and havent ever been able to stand him. I do respect him, but I have to say he really is pretty immature, and seems to have an over compensating fake self esteem that intimidates everyone. Anyway, all was good for him, my mum and my baby brother. They even bought a nice matrass in september with money his mum gave us especially for that, and he bought me a computer for uni even though I have one (which was really surprising).

but one night my parents went out for diner, and he apparently told my mum she was this and that which he didnt like (like extremely irrational stuff, he told her she never laughed and never smiled though HE's the one who never laughs at her jokes, and who doesnt show any interest to whatever I have (or she has) to say), and he wasnt sure if he still loved her. Then like a week later they went to see a couple therapist, and he said horrible stuff. Ever since then he never talks, he's become horrible, he's bearly ever here, spends hours in the garden just sitting on the chair on the weekends, he isnt even nice with my brother who seems angry with him now (he's 20 months old but he really shows his dislike towards him cause my stepdad is like really tense even with my brother, my brother didnt even want him to feed him he asked my mum to do so). So yeah basically he comes home late, sleeps his back to my mum, doesnt talk whats so ever, doesnt say hello or goodnight to me, and just says it in dry way to my mum,he bearly spends any time with my brother anymore, he goes to bed really early and just watchs documentaries on his tablet, he DIDN'T EVEN GIVE ME AN 18TH BIRTHDAY PRESENT (I dont care so much but COME ON,he's been living with me for more than 11 years for god's sake), and yeah he's just getting on my wick.
He's so irrational and he act's in such an immature way it's bearly believable.He's ridiculous and I can't stand him. But I feel bad for my mum and brother.

do any of you have a clue what he can have? He apparently did the same thing 6 years ago (I cant really remember), so I thought it could be some kind of bipolar disorder cause I had seen one time that you can be depressed every 10years and stuff but I dunno maybe I'm mistaking.
or maybe is he having that crisis you have when you're like 40-50 years old? He turned 40 on the 26th of october (2014).

anyway I just want to understand this a little cause right now I want to shout at his face and hit him aha.
What should we do?
We are going to see a yogi tomorrow called Amma, my stepdad knows we are going somewhere, and we are both missing school and work but he doesnt even care enough to ask where we are going!
My mum thinks he's doing everything so SHE says she wants to break up, but she said she would never give him that pleasure.

anyway, I cant see if this message is long cause I'm on my phone, but I guess it is, so thanks a lot for reading, and please share your experience, or whatever you want to answer to this. I think i also needed to get it off my chest a little.

Keane

I hope you feel better now that you have gotten that off your chest. it is hard to say what is wrong with him. he certainly sounds like his mood is being affected by something but he would have to be evaluated to find out. he could be suffering from depression which could come and go. it is nice that you are old enough to get away from him. welcome to psych central.