At the early stages in a relationship, what you say to him should be a little less full on than what you would say later. In the beginning you can say things such as "I really enjoy spending time with you." this will let him know that you like hanging out, without sounding too serious. Saying something such as "I love your cuddles" lets him know that you are enjoying the simple physical acts that you are sharing with each other.
When you guys become more serious, you will want to say things that express your feelings on a deeper level, but are also cute, funny and express your personality. A simple "I love you" can say a lot but it is used too often these days so something else will be more unique and make him feel special.
Try to sound sweet but not too cheesy in the things that you say to him. "The best thing about me is you." This is quite a simple thing to say but it shows him that you care and that you don't want to lose him. "You're the ketchup to my fries and the sponge to my bob" is sweet and also funny and guaranteed to make him smile. When you are lying with your boyfriend or cuddling him say something like "This is the best place to be, I could stay like this forever." This will let him know that you are very happy and comfortable with him.
Only say this if you are certain that the two of you are in a long and committed relationship, otherwise using the word "forever"might freak him out. Telling him "You make me laugh, when I don't even want to smile" is a winner because it will let him know that, being with him makes all your problems go away.
When you are apart from one another, send him a cute text to brighten up his day and let him know that you are thinking about him. Texting him something like "Sweet dreams, wish you were here" will let him know that you miss him when you are not together. Be sure to put kisses on the end of you texts and emails to him.
Try and personalize the cute quotes to say to your boyfriend based on your relationship that you share. Using pet names and "in jokes" that the two of you have is better than plucking something sweet from thin air, modify the cute quotes to suit you. Showing your boyfriend some love is important but don't do it all the time, wait for him to say cute things to you also otherwise the relationship might feel quite one sided.