Fratenal twins are no more alike than non-twin siblings, except for in age. They look different, have different interests, have different strengths, and develop at different paces. I'd bet that, since one child was 3 months slower at crawling, walking, talking, that he's probably about 3 months behind on readiness for reading and all. I wouldn't necessarily hold him back. As they get older a 3 month difference won't mean a whole lot (although it still does now).
It's still only about midway through the school year. As a former kindergarten teacher, I know that at this age their "aha moment" can come at very different times in the year. Some get it in September or October, some after the Christmas break, and some not until spring. There are a few who don't hit it until summer. And it really doesn't have a lot to do with age. So I would say patiently work with both kids and let them help each other. Help your son who doesn't seem to be up with his brother on school things to find what he is strong at. He may be already feeling inferior because his brother is doing all these things he isn't. But he's great at some things too, right? And it doesn't really matter if its a school activity, or if he can tell great stories (which is an indicator of creativity and intelligence anyway) or he can throw a ball better, run faster, or ride his bike better. Help him see what he's good at and praise him for every effort he makes. Do the same with his brother.
I wouldn't hold him back based on what you said. But you still have four full months in kindergarten to see how things go, too.