C.D.
You started with a sippy way too late is why he won't drink out of it. I started giving all 4 of mine sippys at 6 months, two handled ones easy to hold for little fingers. You just have to be strong and keep trying don't give in to him and give him a bottle or you will never break him. If he doesn't want the cup don't give him anything he will get thirsty and he will scream but you are having another baby and you can't have two on a bottle at the same time. You have to be the one in control he will learn that throwing fits get him everything and you don't want that. What are you putting in the sippy? If it's milk he probably won't take it or formula why restle with a cup when he gets the same thing in a bottle. Trust me I have two friends both boys are 4 and still on a bottle and not potty trained because they give in too much. And unlike my son isn't in preschool, my son gave up his bottle at 13 months but my girls were almost 3 every kids different but you have to lay down the law. Sorry so harsh but the next baby introduce the sippy at about 6 months when they are sitting in the highchair it will be a much smoother transition. Just a side nota after reading all the comments, the one rule in my house is there is no walk around bottles at all. My youngest is 19 months and I never let her walk around the house with a bottle I always gave her her cup. And only bottles for nap and bedtime, and another tid-bit you will never bottle break a child if you put anything in a bottle other than water, milk, or formula. Good Luck
C.