I think that's a wonderful thing...being foster parents!!! :)
My baby, now 16 months and 29 pounds, has always been in the 95-98 percentile. He could always eat alot and still can. He was breastfed, which I'm told is one reason he is a bigger than average baby. However, I dont know how it compares to formula, but at 10 months, he could nurse for 15 mins every 2-1/2 hours-3 hours and would eat a stage 2 baby food and literally just about any amount of "grown up" food (mash potatoes, veggies, whatever we had soft) that we'd give him. And I too, was quite amazed that he could eat a whole banana at a time.
To help lesson the calories a little, we would mix baby cereal or rice into his baby fruits or veggies. It seemed to work for a bit, where he'd eat a little less of the other food. BUT I wouldn't worry about him being overweight. Our pediatrician always said he was healthy, and growing proportionately (i just drew a blank as to the correct spelling of that), and not to worry about his weight until a few years from now.
I'm sure, just as others have told u, that once ur baby starts running all around, in time, he'll thin out a bit. I love my baby's rolls. He's as happy and healthy as can be, and that's all that matters at this age. So don't worry one bit at this point.