Hi A.,
What a wonderful mother you are! I am so proud of you and happy for your son that you have taken breast milk so seriously - facts would suggest you have made him a much healthier little boy!!
I nursed my daughter until she was 33 months (1 time a day at that point). She weaned herself and it was a good and beneficial experience for both of us. And, she's been healthy as a horse and smart as a tack :-) Your milk changes as your child changes and there is always benefit to it. I think you have made a great plan to continue through this coming winter as that seems when germs are worse AND your baby will get that many more months of perfect liquid for him.
My daughter started reaching for my water glass at about 5 months. So, with advice of the lactation consultant, I started letting her sip out of it. She was delighted! So, I gave her a sippy cup right from the start. I think I would try that! I started with the kind from Tupperware, they aren't spill proof and don't require sucking to get it out. For my daughter, all along, "big girl", etc has been a big motivator for her - so you might make a big deal out of that.
I don't know that there is a perfect age to be done with a bottle, but I do think that the sooner your progress towards something you don't have to take away, the better. I think the cup would help with that.
You can't give him too much breastmilk, so if it's 4 times a day, wonderful! When my daugher was that age, I still nursed her probably 4-6 times a day. What I did at night was when she was finished, I would read her a book, then go to sleep. That way, it helped not to have her "fall asleep" nursing. I didn't wash her mouth out with water or anything at that point and she had no problem with the breastmilk causing mouth rot or anything :-).
I didn't and still don't do much juice for her, there is so much sugar in it! She now drinks milk and water and is perfectly happy with it!
Best of luck to you, you are making good decisions and your son is the one who is benefiting! Keep up the great work!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!