Responding to your boyfriend's expression of love can be heartfelt and genuine. Here are some ways you can reply:
"I love you too, more than words can express."
"You mean the world to me, and I love you deeply too."
"Thank you for loving me. I love you with all my heart."
"You make my heart so full, and I love you too, incredibly."
"I'm so grateful to have you in my life. I love you too, darling."
"Your love brings so much happiness to my life. I love you too, sweetheart."
"I've never felt this way before, but I love you just as much."
"You're everything to me, and I love you too, my love."
"I'm blessed to have you, and I love you deeply as well."
"I love you more each day. Thank you for being in my life."
Feel free to use any of these responses as they are or modify them to fit your personal style and emotions. The most important thing is to be sincere and express your feelings genuinely, just as he has done.