Hm, not sure why your doctor thinks there's a big rush on this. My baby is a year and a week old and he's never drank from a sippy cup, and life is good. And I think it's not really recommended for babies to drink juice so early, unless perhaps diluted halfway with water, but in any case, juice is another thing we've been in no hurry with. He does eat some fresh fruits, but not juice, because we're not really a sugar consuming family, so I don't want to get him hooked on very sweet tastes very early. But back to your query -- one thing you might try is a regular cup, or take the lid off the sippy cup and try to get him to drink from the cup part alone. (This is what we've done.) Granted, it takes patience to sit and hold the cup for him, and tilt it a little bit for each sip, being careful that he is actually doing the sucking and gravity is not forcing the liquid in too fast. It helps if the cup is clear plastic or translucent in a light color, so you can see the liquid moving in it, and if you put very little in it at a time. And be prepared for the first dozen or more sips he takes to come straight out again, until he learns to move the liquid back in his mouth and swallow. This has worked for us because he sees us drinking from cups and is interested in learning a new process. In fact, our challenge had been how to get him to drink water once he started solids. Before that he was exclusively on breast milk (both nursing directly and breast milk in a bottle at child care) and just refused to drink water from a bottle at the table. Once we introduced the regular cup, he would drink water out of it and be very proud of himself! But this didn't start until close to his first birthday. Maybe some people think sippy cups have an advantage because a child can drink "independently" from them, but at this age, I'm happy being involved in my baby/toddler's feedings. He certainly knows when he's hungry and when he's had enough, and I respect that, but his world is all about play and exploration now, and I don't necessarily want him to treat bottles, cups, sippy cups, whatever the food delivery system is, like a toy. Also remember all those warnings about how leaving a child with his own juice will be bad for his emerging teeth. ANYWAY I hope trying the regular cup is helpful for you, but whether it is or not -- the important thing here is that every baby is unique in his preferences and his development. (Incidentally I had a friend who advised me to start baby on a straw, because that worked for her kids, but my baby could not have been less interested.) Enjoy feeding times and all times with your baby, as every stage he goes through will pass quickly, and don't feel pressured to conform to what your doctor THINKS your baby should be doing at any particular time. Only baby knows for sure what works for him, and there really is no worry that he will end up in seventh grade without being able to drink from a cup!
(PS That mom who wrote about screaming himself to sleep a few nights -- I cannot imagine why that would be a good idea. It seems to me that when an adult, whether it's a doctor, a mom, a grandma, whatever -- when an adult makes this kind of timing decision for a child whose only means of talking is making sounds and crying -- then it is a RANDOM decision, usually based on what is convenient for the adult, or some misguided idea that all babies should be on the same developmental schedule. Babies and toddlers really are capable of making these transition decisions themselves peacefully, and they WANT to transition, because it's fun for them to grow and change and try new things. I think it's our job to make their little lives less stressful than ours were, so they can grow into self-confident and successful adults.