I don't have any experience with immersion schools but I would think about your DS's personality; is he a mature 5 year old? Do you think he would be bored in Kinder when the class is doing not so academic fun activities.
DD6 didn't learn a thing in Kinder but she also never complained and loved the fun aspect of it. However, because she was not challenged and is very verbal, she drove the teachers nuts sometimes with her shenanigans. She also has excellent processing speed so she always finished her work lightening fast and then found it hard to wait for others to catch up. But, she still missed the fun parts of Kinder during the time she was pulled to the 2nd grade class for reading. Because of this, I'm glad she had the experience of kindergarten, the fun activites and classroom behavior type learning.
After I shared her test results with the school (she is a DYS), she started going into 1st grade after her kinder class ended for the day. Aside from the writing pratice, it was still not an appropriate placement. The 2nd grade teacher also informed us that she should be in a higher reading group. My point is even if your DS is 1st all day, you may find that he needs additional subject acceleration in math and reading (with accomodations in writing requirements) very quickly. I think the key is how flexible your school will be with that notion and whether they will be continously assessing his capabilites throughout the school year.
I was unable to advocate for a grade skip for her next year (district very anti-skip) but I am going to do some serious advocating during the school year to ensure her pullouts and growth are being monitored to make adjustments during the school year. Luckily we did not have any problems with DD having to go to all these different classrooms.
