WE went from bottle to sippy cups. We transitioned fairly quickly through the various types of sippy cups until we came to one that was most like a regular cup. He still wanted a bottle at night for comfort but we still tried to give him a sippy cup. The night time bottle was difficult/
That is a lot of milk. Wow. Does he eat enough food?
Try to avoid a bottle of milk at bedtime for your son's teeth. We struggled with that as well so I understand. At around a year and a half, we put water in his night time bottle (actually then switched to a sippy cup) instead of milk. Our son had the most difficult time for a while and we endured the tears...oh so many tears, because he wanted his milk. He got used to the water eventually. I just had to be strong. There were a few times I gave in because I just could deal with the tears. Our son is relentless. I regretted those few times as it made it more difficult. I am only human.
I really wonder if your child is getting enough calories if you have to give him the diluted stuff more frequently. I didn't have that problem with my son. That sounds difficult.