Random advice. We have a 2.5-year old.
1) Create a buffer of reduced distractions for an hour or so before bedtime. That means no TV or other stimulating excitement before bedtime. You must set the atmosphere. Quiet time precedes bedtime.
2) I actually avoid sugar, close to naptime/bedtime. I consider it crack cocaine. No sugar within 2 hours before bed.
3) That sippy cup should contain milk or water, not juice or soda.
4) Create a familiar routine that slowly, inexorably, heads to bed. Do the same thing, in the exact same order, every single night. Bath, jammies, bottle, toothbursh, get into bed, songs/stories.
It may feel like you're being a military drill sergeant at first, but toddlers find comfort in familiar routine.
5) Staged crying responses. If the fuss starts immediately after you leave the room -- or even before you leave the room. Give it 30 seconds, then return, comfort and love, explain the routine in simple terms, leave. If the fuss continues, wait 60 seconds, return, comfort and love, leave. If the fuss continues, wait 2 minutes, etc.
The basic points you are trying to communicate is (a) mother and father are still there, but (b) you aren't going to give up on putting him to bed.
And sometimes it's just plain hard work. For a while, we wound up camping out in the room for an hour or more each night, as our daughter got used to the new routine. We would be in the room, during this time, but would not respond to efforts at play or interaction... being as boring as possible, breathing deeply with our eyes closed, pretending to sleep, ourselves.
This is very good advice. The routine is especially important. I have a 2 year old and 2.5 year old... now, sometimes they will complain and cry when we say it is time to go to bed, or during the routine (because they know bedtime is coming), but they don't usually cry when they are in bed anymore. If my 2 year old daughter cries when we say goodnight and leave the room, then one of us will go in and reassure her that it is time to sleep... I usually make her agree that it is night-night time, and then she's fine. I suppose that wouldn't work well with a toddler who isn't talking yet though, unless he at least acknowledges your questions with a head-nod or something.