Libby, breathe.....
At 8 mos. I don't think Zephyr will be confused with breast milk in a sippy cup. You can put some in it. Water is great stuff, we all need some every day. It will work well.
Apple juice, white grape juice are great. We're talking about small amounts and you can dilute them with water if you're afraid of the sugar contents, but use juice from concentrate with little or no added sugars.
With regards to the standing and walking... great news!! Yes, he's emulating his big brothers. No need for concern, just watch him to protect him from falls and his other brothers in their play. My oldest was walking on her own by 11 months, so pulling himself up and walking around furniture at 9 months is on target. You will have your hands full, but you knew that already. It's going to be nothing but an adventure, some totally awesome days and then some very exciting and scary moments, but you will all survive. Enjoy!!