Hi, A.--
First, I have two boys, the youngest has a speech delay and just finished his first year of kindergarten with a speech IEP. Second, I'm a teacher with a Master's in Special Education and time in a special day class, K-3.
Now.
:)
You're worried about your son being confused as his sister's twin if they're in the same grade--I had sisters who were in the same grade because their birthdays were so close to each other. Once people understood everything was normal.
Your husband is worried about him not being with other boys his age--that's not exactly a concern as they will all be within a few months of each other. Though some may still be in diapers I would imagine that that won't last long. My youngest started the state pre-school in diapers but was out of them within a few months. Not sure what the difference was between seeing his classmates use the bathroom over his brother, but dang--at least it worked! :) And I do know that his other classmates didn't regress just because he was still in diapers.
The chance to spend time in a regular kindergarten class seems to be a benefit and I would definitely take advangtage of it unless it proved to be a problem (which I can't imagine it would).
Does your district have a different kindergarten class specifically for kids in your son's situation? Our district has a pilot kindergarten set up to give late-birthday kids with a special need (mostly speech) a chance to prepare for regular kindergarten. Nine of the 20 kids were November birthdays and I believe most or all of the students in his class had an IEP. I helped out in the classroom once a week and saw huge growth in all them. They are such different students than at the start of the year that I know they'll all excel in kindergarten this year. I'm excited to see how Alex does this next time around.
If your school doesn't have such a program then I would lean toward keeping your son in the pre-school program one more year. If you're sure it would be best to send him on then I might ask the district to put him at a different school than the one he would normally go to. This is a decision that completely rests on how you feel about it. I felt good about my son starting the second round of kindergarten with new friends while moms of the other kids in his class were excited about keeping their child at the same school.
What I liked about this pilot k so much was that they were given more time to explore/learn without all the extra pressures of having to obtain the standards and benchmarks for kindergarten. It was more like kindergarten of old and the kids learned while still having fun at school--an important base for the rest of their education. Their teacher was experienced with this "breed of student" and as such she was able to help with their speech therapy needs. She was a great asset to their education this year.
I can't emphasize the importance of making sure your son doesn't feel like he's being pressured at school--especially at this age. It's much better to spend extra time now to get the basics down than to have to do it later. And if he's a bit older than the rest of his classmates it's not going to be by much and for a boy that's just a few more months' maturity to aid him in everything from academic acheivement to sports to social aspects. :)
Good luck with the decision--hopefully I haven't given you too much to think about that I just confused you even more.
M.