You know, everyone have different speed of emotions, in term of emotional attachment with one person. One person can fall in love quickly while other take longer to fall in love with someone. Lot of time in my personal experiences with my past relationship that she always said "I love you" first before I finally says "ILY" back month later. Sometime she just spilled out that she didn't meant to say it that way.... It's more of compulsive reaction that she need to say it. After she said that, I told her that I'm glad that she love me, then I told her that I may take longer to say it back, because I'm very cautious and want to be sure we are truely in love together. I don't want to get any blind sided being in love too quickly.
There are time to time that I hate went somone said "ILY" first, I was like "oh no", but after I explain to her that I can't force myself to say it back to her, or for me to "fake it" to say I love you back, but I told her that I really enjoyed being with her. Told her that I'm a slow emotional attacher. She understood... As relationship carried on, she been waiting and waiting, hoping to I say I love her finally... which I did month later after she said it first. Then she jump in joy.. cuz she was worried that she maybe not good enough for me.
So, for you, he maybe in love with you suddenly, but you can tell him honestly how you feel about him, just say that you still enjoying be with him, but falling in love with each other at the same time? ONe person take little longer than other person... and let him know that. Let him know that he can't expect you to fall in love at the same time he does. You know. So being honest with each other will bring two hearts closer together.