Your baby may not be ready for a sippy cup yet. Maybe if you continue to offer it, without pushing it on him, eventually he will use it. Also have your 5 and 8 year olds use one to "show" your baby how. Plus they love to imitate their siblings. If you are nursing, he is getting plenty of water/liquid and the best source of it too. Here are some suggestions on how to try and get your child to use the cup:
What should I do if my child refuses the sippy cup?
Babies have all kinds of reasons for rejecting sippy cups. And of course, there's no law saying he ever has to use one. Some babies graduate from breast or bottle straight to a regular cup. If you'd rather your baby learn to use a sippy, though — for convenience, or because you think it'll make a good transition for him — moms have used these tactics successfully:
• Dip the tip of the sippy spout into the milk or juice and then give it to your baby. He may just need a hint!
• Show your baby that the spout is like a nipple (it needs to be sucked on). Try touching the tip of the spout to the roof of his mouth to stimulate his sucking reflex.
• If he drinks from a bottle, give him half of his formula in the bottle. When it's empty, switch to the sippy cup for the second half of the feeding (continue to hold him as for bottle-feeding). Or put a bottle nipple (without the bottle) in his mouth and after he starts sucking, replace it with the sippy spout. Some moms have even had success telling their baby it's time for the "ba-ba" — and simply substituting the sippy for the bottle.
• If your baby sucks on the sippy spout but doesn't get anything, try taking out the valve that controls the flow (if his sippy cup has one of these and it's removable). It'll be messy at first, but once he learns to handle the cup you can put the valve back in and he'll probably be willing to suck harder for his drink. Or make an extra slit in the valve to allow the liquid to flow more easily.
• Work backward. Teach your baby to drink from the sippy without the lid first. Put just a teaspoon or two of liquid in at a time and help him raise the cup to his mouth. After he gets the hang of that (and sees that there's liquid inside the cup), then put the lid on (without the valve, if there is one). Finally, put the valve in and let him take over.
• Offer your baby a straw. Some baby cups come with built-in straws, and some babies find these easier to use than a spout (though others will look at you, clueless). If your baby does get the hang of sucking from a straw, he may then be better able to handle sucking from the spout.
• Switch beverages. Some babies will drink water or juice — but not breast milk or formula — from a sippy. Sometimes it's a matter of association (milk belongs in a bottle or breast). Some moms have had success waiting to introduce the sippy cup until their baby is ready to start drinking whole milk (at age 1).
• Show him how. Get yourself a sippy and let your baby see you drink from it. Or have a sibling drink from a sippy in front of the baby. Sometimes all it takes is a little sucking noise (make it when you give him the cup) to inspire a baby to start sucking. One mom sat down with her cup of water and her baby's sippy cup of water. She drank from her cup, then helped her baby drink from her cup. Then she drank from the sippy and offered it to the baby for his turn — and voila.
• Shop around. There are all kinds of sippy cups, with all kinds of spouts. There are even bottles that transition from a regular bottle to a bottle with handles to a sippy cup with a lid and straw to a regular toddler cup. Sippy cups aren't too expensive, so it's worth letting your baby test-drive several if the one in hand isn't working. (And as he grows to toddlerhood, you might let him pick out a brand-new kind himself, just for fun.)
Good luck.