How do I get my boyfriend to work on his avoidant attachment?

My boyfriend and I have been together for about 6 months. At first things were great. He told me that I was “The One”, he did romantic things, messaged me every day, we spent time together, not as much as I wanted but it was still good.

But recently I feel like he’s pulling back. Like sometimes he takes ages to reply to my messages and it drives me crazy cos I want to message him but I don’t want to double-text and look needy. But inside I’m like waiting and when he does finally reply it’s such a relief yet I feel angry at the same time. I try to act cool about it, but sometimes I also take my time to reply so he knows what it feels like…