Why is my boyfriend acting like this?

We’re each others first love and first everything. We started dating at 13-15.

We’re 17 now and had a 2 year fwb but dated other people.

We started dating in November. But we’re talking since august, and he had sex with an ex without telling me in october. (kind of cheating, we weren’t offical. it has created trust issues regardless.)

Everything’s been going great, until about 3 weeks ago. He’s wanted to hangout everyday, but has lost interest in my conversations, and sex KINDA, although it’s still enjoyable. He isn’t as excited to see me as he used to be.

Of course this make me extremely concerned. So i brought it up. and he said “I’ve been feeling distant from everuone lately. My mom said i’ve been acting different and i said i don’t know. It’s not you. I want to fix it and turn it around. I’m sorry you feel that way i want to fix it. We can go to lunch tomorrow and talk about it.”

It gave me sort of reassurance but i’m also worried. He’s been stressed about a lot of stuff lately in his defense. 

Is he getting tired of me, is it depression firing up, or is is stress??

I need help