We also have one of those in my son's class. Likewise, he is one of the younger boys (just like Nick) but is bigger than Nick and a few others. Unfortunately, as he is one of the younger ones that started just before his third birthday (like Nick), he will be around for two more years with my son. I have been less than pleased. And, to boot, he is now everywhere we go, even in the basketball program that we just put our son in. It's frustrating.
Anyway, after much thought and discussion with my husband, we have agreed not to specifically request that the two be kept apart next year. Why? There will always be that one kid in class that acts out, physically or even just disrupting a class. There will always be one kid that is nasty, that either ignores how he has been raised or who isn't raised with attention to manners. (The former mentioned because, hey, I know my kid has acted out in public before despite being raised with attention to manners!)
Will I step in if I hear any more stories of pushing or hitting? Definitely. I won't stand for my child being bullied in preschool! But until that point, I will continue to teach Nick that we don't push or hit or call names and reinforce that we use nice words and not ugly words.
Oh, how other children affect our parenting.