Basically think of anything nice that you would like to hear from him, and say that to him. Of course, obviously things like "you have such a cute little nose" wouldn't sound as nice for a boy as for a girl, so you may want to think of gender-neutral things like "you're what I've always dreamed of".
Chat with our AI personalities
You could start off saying how much you really love him and even though he knows it, he does still like to hear it now and again. Tell him how much you want to be with him and you never want to be apart. Say that you will never be able to live without him. If you really did love him then you would know what to say and it would just come naturally to you.
The sweetest thing could be I love you or You are so romantic, loving, (pretty much good compliments you could say). Something complimenting him about being with you or stuff like that.