My dd does not take a sippy cup! I give it to her with some water in it, but she just plays with it. It keeps her entertained for quite awhile! Anyway, I know that they are supposed to get most of their nutrition from breastmilk still, which is why I only give her a certain amount of baby food. There are times where I think that she would take more solids, but I want to preserve her nursing later. Also, on the times where she acted like she wanted more, I gave her more and she only took one or two bites and was finished. Sometimes, they just need a chance for their mind to catch up w/ their stomach. Well, good luck, and I hope that you figure out something that works! It sounds like it is going pretty good, so I'm sure it will be fine.
By the way, do you give him any solids yet, like soft fruit pieces or anything like that? I was wondering when I could start dd things like that, and I'm not sure what to try first. She already eats cheerios.