The Nuby sipper cups are great- no valve, and they have a soft silicone sipper top. Another good option are the take and toss cups. They are inexpensive and really do work well. I'd stay away from the straw ones for now since he isn't getting the hang of it yet. I think it is harder for them to drink through a straw. Keep working with him on getting him to hold it himself. As far as how much milk/formula, etc., it is basically an equal ratio. My doctor told me to give my daughter 24 ounces of milk a day once she turned one. He should only have whole milk (also called vitamin D), not 2% or 1% or skim, because they need the fats for brain development. As far as transitioning him to milk, usually they say start at a year, but if you start a couple of weeks early it won't hurt. If you have any milk sensitivities in your family you will probably want to transition slowly. It is probably best on his stomach if you do 1/2 milk and 1/2 formula (or 1/3 milk and 2/3 formula) at first and every few days or a week, increase the amount until he is just taking milk. It's not rocket science- just increase it slowly and see how he does. If he starts having lots of diarrhea, or really bad gas, or vomiting, slow it down more and talk to the doctor if it doesn't go away. If you had him on cows milk based formula which is most of them, (you would know if you had him on some specialty formula) then he should transition just fine to regular milk. You'll do fine.
One more thing about the sippy cups- if he decides to chew on the Nuby sippy cup, then you could try the Playtex ones, they work great.