Your concern is a common one, K.. But there is really such a huge range in children's development that while we can talk about what's "average," or within "norms" on testing, it's not a wise or admirable lens through which to view our kids.
This is not exactly the answer you are asking for, but it's terribly important: Every child is an individual. Almost every one is way "ahead" or "behind" in some areas: mathematical conceptualization, verbal skills, writing, spatial development, fine or gross motor skills, reasoning, artistic expression, etc.
If your son responds well to coaching/tutoring, super. He may drink it up. But please stay attentive to whether it's becoming a grind for him, because that will only work against the pleasure he takes in learning, which is a natural gift kids have.
I hope your focus with your sons will be to foster their enthusiasms, and to share your love of whatever it is you love, so they can stay in touch with what that looks like and how it feels. If a child is slow in some educational area, it really will make no difference in how great a person he turns out to be, and in fact, most kids do catch up in areas where they may have been "behind" as soon as they are ready to advance.
I've worked in education in one way or another for decades, and have seen how much damage it does to kids to try to force them into homogenous classrooms. Maybe half the kids in such rooms do well, the other half are stressed to some degree – by boredom or anxiety – and some of those kids shut down when they can't take it any more.
I understand that most second grade teachers are more into content, like imagination and age-appropriate expression, than they are sticklers for spelling, punctuation, or even grammar. Much of that learning is still to come, and much of it will be learning through parents reading to their kids, doing projects together that require reading, and even conversation. As children read more, they will often become more familiar with the spelling of words.
English is complex and it takes many kids years to sort it out. Both my favorite English teacher in high school, and my husband, who writes science curriculum, are terrible spellers. Who cares? They are both successful and wonderful people.