<human>: Can you succinctly summarize the text in 1-3 sentences?

My boyfriend and I have been together for 2 1/2 years, with a few months split inbetween.
We got back together a few months ago and it's been going quite well. 
I'm very happy.
However in the past few weeks, he's not seemed very happy at all. at first I thought it was me, but his friends and work colleagues have also noticed his low mood when he is around them. We give each other a lot of space to do everything with friends and family sepstately, so I don't believe it's me smothering him.

I'm really worried about him. He absolutely insists he's fine, and gets angry when people ask if he's alright, and feels we are pestering him.

I would agree that if he wanted to talk about whatever is causing this, he would. But he won't talk to anybody about it, not even his parents. 
He's been out of character for a while, so it's not just been a bad day at work making him feel this way. It breaks my heart to see him so down, and it hurts all the more because I don't know how to help him because he insists there isn't anything wrong.

What can I do to support him through this difficult time, and make him feel safe and comfortable when speaking to me and when around me? I just want him to know I am here to support him through everything, but without being a pain in the butt and pestering him about how he's been behaving differently.
<bot>: boyfriend is clearly unhappy but won't speak to anybody about it and insists he's fine and that everyone should stop pestering him. How can I support him with pestering him?