Me [18F] and my bf [18M] are struggling because he cant get over of his ex. What can we do?

So first of all english is not my first language so i’ll try my best to explain the situation.

Me and my bf have been dating for 1 year and things went pretty well for us. We were a loving couple and we always work things out together.

Recently, things went pretty stale since we havent done anything new, everything feels repeated. About a few weeks earlier, he started to feel melancholic and started remembering about his previous relationship, how things were and his feelings back then start to arise.
(his previous relationship was with my now ex-bestfriend and things ended on bad terms because she was emotianally unavailable)

He was baffled since he thought he was over her before dating me and he would never date anyone until he got over his ex.
(we dated 4 months after their breakup and if youre wondering if i were the third person, i was dating my ex when they broke up so there’s no chance)
He got scared of the thought of her and would try his best to ignore it, focusing on us and our experiences instead.

It doesnt last long. Things got worse now. He would feel blue most of the time and cant really feel and express love. The only thing he can do is being nice to me. He doesnt know if we should continue or not.

I was devastated. It’s like a virus. I cant think of anything else except my bf and his ex when i pay attention to the lyrics, to the point that i have to listen to songs that arent about love or songs that in another language so that i dont have to visuallize.

I afraid that he might got depressed again (he used to have depression and anxiety 4 or 5 years ago). I really love him and im willing to help him out. I read articles after articles, suggesting new things to do, new stuff to try and be nicer, kinder to him. We talked to our friends about this and have decided to give ourselves some time apart to focus more on ourselves since we was quite dependent on each other. We have also decided to go back to our hometown together this weekend (we moved to the same city for college)

It has been 2 days after we decided to put effort in our relationship to work things out. I have tried my best to work with myself, learn to be thoughtful and listen to his worries instead of getting emotional and scared. I have also been fixing my posture and try to make myself look good to impress him.

I can see he got some progress also. He doesnt tell me “it’s too soon” when i express my love to him. He talks more about “us” in a possitive way (seeing a picture of two cats hugging, “us” “cbu”)
He starts to text me “ily” or “thx”
(Vietnamese word is “thương”, we wrote “thx” for short. It is a different kind of love, people use it when they really understand each other and would take on the responsibilities that their relationship demands).

Hours ago when i tell him i cant listen to music, he starts getting sad and being pessimist. He often be like this when it’s after 12am and become all fine and normal the next day. The cycle repeats.

I know that it has only been 2 days and that things would improve if we were to see each other irl (we havent seen each other for 5 days) but i couldnt help but worried about our mental health and the future of our relationship.

What can we do to get back on track?

TL;DR: Bf gets flashback of his previous relationship out of the blue and it’s taking a toll on our mental health and putting our relationship at risk.