This is really common with babies that go from breast milk to formula. Breastfed babies usually have mustard yellow, sometimes orangish or reglar looking bms because nutrients from mom are more evenly delivered, unlike nutrients gleaned through a formula. So don't worry, just yet. The iron especially and other added vitamins and nutrients found in formula are the culprits. For some babies, the intense amounts of these additives can mess up their bms and they just have a really tough time digesting. You could try a special formula for lactose intolerance etc, but you should note, that whether its soymilk, nutramigen, or any other formula, they still get the green bms.
Since you didn't mention whether your baby seems to react adversly to the formula, I"m going to guess he is not intolerant. Usually, those children with allergies get really severe diareah, tummy cramps, gas, and sometimes even rashes, so you'd know.
My oldest was the same. He did not do well on formula and so with the okay of my pediatrician we just went to regular whole milk and began using a baby formulated multivitamin. I think he was about 10 months too. He improved almost overnight..hardly a green bm to be found.
Be aware though, the liquid vitamins for babies also have alot of iron which means they might get constipated. So to ensure normal bms using this scenario, be sure to feed baby lots of nice fiber like pears, dried fruits, even prune juice will do the trick.
If you decide to just try the milk thing, be sure its whole milk because babies need the extra fats for brain development, and of course check with your pediatrician to be sure the baby is ready nutrient-wise.