Yes offer water with each meal, don't worry if he doesn't drink any for ages yet. It's just to get him used to the idea and the process, not to get a volume of water into him.
Re milk feeds, milk should still be the main source of nutrition for a good few months, food is in addition to this. I found that milk feeds dropped of their own accord as my DS got older, I'm assuming the same is true if you use formula.
Any frequency of poo is fine! As long as he's not constipated or got diarrhoea, or anything else unusual.
The NHS website says that you should aim to be giving 3 meals a day by around 8 to 9 months. It also suggests giving finger food too, if you're not doing BLW. As he's 6 months old you can give him pretty much anything food wise, apart from honey and obvious choking hazards like whole nuts. So you can widen the range of what you're offering, and there's no need to introduce one thing at a time, unless you've got a family history of allergy.