i totally agree with all replies so far and I would say go for it. We were in a very similar position to yourself in that we immigrated to Bexley from Australia 2 years ago. At that point our DD started in year 5 in September 2008 when she was only half way through year 3 in Australia. At that point we had no idea about the 11+ and when we first looked at the practice papers we immediately thought that there was no way she would be able to cover the ground necessary before the 11+ exam the following September. To cut a very long story short by May 2009 we re-assessed the situation and found that she had progressed so much at school that it was worth giving the 11+ a go. We hired a maths tutor because our main concern was that she had quite a few gaps with her maths because of the move and she needed to build her confidence. This was the best thing we could have done as her confidence improved remarkably and our thinking at that stage was that it didn't matter if she didn't pass the 11+ as she was getting a lot out of the tutoring regardless. We were very excited when we found out in October last year that our DD passed the Bexley 11+ (she didn't sit the Kent test as we didn't think we had the time to get up to speed with the NVR) and she is very excited about starting at Chis and Sid in September.
I guess what I'm trying to say here is not to worry about the fact that your son was a little behind when you returned from Australia as he sounds like he has already caught up and will continue to make up ground. If he has the potential you have plenty of time to prepare him either through the use of a tutor or using the practice papers and associated guides that we found very helpful. A tutor will also give you a good idea as to where your son is at and fill in any gaps if there are any. Also, how did your son compare to his class mates in Australia? This should give you a bit of an idea as to what his potential is.
What I can also vouch for is that every year children with the potential to pass the 11+ do not as they haven't had enough preparation even though they have the ability and many children with less ability do pass because of the amount of preparation they do. Unfortunately Bexley schools are advised not to promote tutoring or too much preparation but I believe that it is very difficult to pass the 11+ unless you have done a certain amount of preparation.