Is letting your boyfriend touch you breast a sin?

I’m with a non Christian boyfriend so I’m having a hard time navigating sexual boundaries. I know that I would never allow sexual intercourse before marriage but I’m not sure if it’s okay if we make up and let him touch and see my breast. We sleep on the same bed when we travel. We cuddle and sleep while hugging each other fully clothed ofc. One time while making out he tried to go under my bra but I stopped him. He said sorry and I also apologized because I couldn’t give him what he wanted. The last time we saw each other he was very gentle and I allowed him to explore my breast. I allowed it because it doesn’t feel like he’s out of control like last time when he did it in an urgent manner. This time , it just feels like he wants more intimacy but knows the boundaries.

I know and I’ve read in some threads that this is a sin.. but in this modern age of dating. That’s the least I could give him. Please enlighten me. Pray for me. Let me know if I’m wrong and guide me. Thank you vm.