We moved from a British system school (international) to an International US school / with an IB programme choice for senior years.
You will find that in the early years the US system focuses less on reading/writing/academics, than the British system. Not a bad thing in my view at all, by about Year 3 or 4, they are pretty much similar levels. Math covers US currency and they will also cover US history and geography, which is a big downside in my opinion.
If you are going back to the UK , it is probably easier to keep them in a system in which they will fit back in. If you are going to be expats moving around the globe, the IB is likely to be a good choice.
If you want him to start in a lower or higher year, , best to do it now, it is much harder to delay or advance a year if he is already in a school, there is a lot of red tape involved with the government department if you want to do it at a later date.
Not everyone agrees that UK curriculum schools are definitely better than IB or US ones.
There are some very good schools in Dubai, there are also some really bad ones, so do your research before choosing the schools to apply to.