My Boyfriend Won’t Tell Me What’s Wrong

So there’s a guy I’ve been best friends with for about six years and we started dating two months ago. I know him really well and he was always really open about his feelings and everything going on in his life. But since we’ve been dating he’s basically covered up all his feelings. Even though he’s still smiling and making jokes like usual, I can always tell when somethings wrong or when he’s upset. When I try and talk to him about it he denies it or says it doesn’t matter. What can I do to help him? Does he not trust me or am I not doing enough? It’s really upsetting me. Please heeeeelllllppppp.