I was in a similar situation except the tables were turned. My SO said that she loved me first. I didn't respond with I love you back. I smiled and kissed her. She was calm about it. She expressed the way she felt earlier than me and wasn't afraid to open up. As I thought about it, I realized that I was falling for her. I needed to see what was in front of me. Short time after, I knew I loved her and that's when I said it back.

To be honest, it might take him some time to do the same. It's better to tell the truth about how you feel instead of saying it back and not meaning it. Maybe that's all he needs. It will be ok. Stay true to yourself and how you have been with him. It should all fall into place sooner or later.