I wasn't able to get my daughter to REALLY take a bottle, sippy cup or straw cup until she was 13 months (a full month after she decided to stop nursing), so I understand what you're going through.
This is what worked for us, irt nursing during the day. Her room is only for bedtime (including bedtime activites, such as reading) so there isn't much that's too distracting in there. First thing in the morning when she would start crying, I would go in, leave the lights off, take her to the rocking chair and nurse her. I would usually have a quiet conversation with her. Then we would go have breakfast together.
At naptime, we would go to her room and read a book with the lights dimmed. I would then turn the light out and nurse her, being careful not to let her fall asleep at the breast. I would then put her down. When she got up from her nap, we would repeat the morning's ritual.
We followed this plan for her second nap, and bedtime as well.
Also, depending on how you feel about night-time nursing, you could gradually drop the feedings. There are other ways besides the CIO method, so if you're dead set against that, look around. One book I suggest is "Baby Whisperer." I would go in and comfort my daughter, but not nurse. After a couple of nights, she pretty much decided it wasn't worth waking up if she wasn't going to get to nurse.
Also, Nubby has some super cheap sippy cups with silicon tops - just pinching the sippy allows the fluid to flow out. We used these when my daughter wouldn't nurse or drink anything. She loved to chew on it, and while much of the drink would end up ON her, at least some was going down.
As long as he is still peeing, still has saliva and is still able to produce tears, I wouldn't worry about dehydration. At his age, children are still in touch with their body enough that if they're thirsty, they'll drink, if they're hungry they'll eat, and if they're full, they'll stop, so try to rest easy!