when should I start trying to transition my son from bottle to sippy?

That's about when I did my girls, started around 9 months and completely took bottles away at 14 months, I know that's a long transition stage but it was very non stressful for them. I would just give then a cup with juice until they learned how to drink out of it. Then only give then juice on cups, then slowly add formula cups instead off bottles, and woo hoo no more babas.

There is absolutely no need to use the sippy cup to replace the bottle. If you were breastfeeding you wouldn't suddenly start pumping and offering in a sippy cup.

I simply started offering sips of water in a sippy cup when I introduced solids at 6 months. The kind with the really soft spout (Nuby) worked better than ones with a semi-soft spout, but he never REALLY took to it. At 7 months I graduated him to a straw cup, which he got much more easily, as well as started to practice with a regular cup once a while.

As far as I am concerned, formula comes from bottles, other liquids from a (straw) cup. Just like as if he were breastfed, the bottle being a substitute for the breast. I'm not in any hurry to change this arrangement until he's old enough to switch to whole milk, and even then I'll probably keep offering his night milk in a bottle. The sucking itself is soothing, I don't see why I should deprive him of that.

If you really want your child to take his formula from a sippy cup, you have to wait until he really has the hang of it. Formula is still his most important food, and eating should never be a struggle. Nursing from a nipple is how a baby is biologically programmed to get the bulk of his food, food of course being milk (breast or formula) and not solids.

maybe try holding of the sippy cup and use the bottle for juice and formula. or show him how to drink from it with another sippy cuo so he can copy you

You can give him a sippy cup. Don't give him a night time bottle when he goes to bed.