E.R.
My oldest started with a sippy cup around 8 or 9 months (I breastfed, but while he was eating, he wanted to drink like we did, I suppose, and since he never had a bottle we went straight to sippy cup). Although I still nursed him (and did until he was 2), he nevertheless loved holding his cup and drinking his water from it (at 8 months it was the summer and he was thiiiiiirsty). I can't really say from experience about pumping and putting it in a cup (since I only nursed and didn't pump with my older two) but it seems that if he doesn't want the bottle, you shouldn't give it to him (it's probably going to be easier in the long-run since it's one less habit to break). As far as sippy cup content, (and I don't really know about putting breastmilk in it, tho if he'll take it, and you want him to have it I guess you could) the only thing that I'd make sure to be careful about is what you put in the sippy cup...a little juice can be diluted with water instead of straight juice if you're thinking of adding types of fluids to his repertoire. As far as cups go, different babies will have different preferences, but you could buy a handful of different kinds to see what he prefers/what is easiest for him, though just make sure that the handles are large enough for him to hold onto. The packaging will usually tell you the ideal ages for the cups and the ideal stages that the baby is in, so that's a good indicator in general. Personally, we used a Gerber brand with my oldest but my middle did not like it and we used some Target generic brand sippy cup. And kudos to you for planning on potentially nursing him more than a year! That's wonderful! Best of luck!