OK...let me preface this by saying things are changing all the time...and we had Vicky's help.
Having said that, I came over in June 2006 on a spousal visa. Then we had to wait a few months while we found a flat and I got a job before we brought my older children over.
My oldest had to come on a student visa because he was already over 18 and there was no chance in hades he was getting in otherwise. Thankfully I have a rich former mother-in-law that covers his expensive tuition every year...and we covered everything else.
But my older daughter was turning 18 as we applied. We got the flat on 6th November. Same day she turned 18. We were able to get lease and send her application before her b-day. Her visa was approved...and she was actually over 18 when she entered the UK on it on the 15th Nov. And with Vicky's help we were even able to get her ILR in October this year.
But like I said...and Vicky or Garry will correct me if I'm wrong, but they are getting MUCH more strict on applying the rules. Even if you succeed in getting him in on the fiance visa, you have two more hurdles to overcome. If he is over 18 when you get married and switch to spousal then you may have to show that he is still dependent upon you, i.e. a student who is not working or able to support himself. And same thing two years later when it is time to apply for ILR/probationary sitizenship.
The other thing though is that you really have to show reason why it is in the child's best interest to be in UK with you. When they are younger it isn't so bad, but older kids...it actually asks on the application why they cannot live with a relative in their home country.
Remember also that even when you are here on a spousal visa, your child does not qualify for home student fees until you have been here 3 years (and that could change too). Compulsory education ends now at 16...although they are considering extending that to 18. And it is NOT always easy transferring your son into a 6th form or college. I was able to get my younger son into a secondary school, but they put him a year behind...because they were afraid he would lower their GCSE scores if he entered half way through year 11. And my daughter...she had to sit home for a year. Then all she could get into was the community college because she did not have GCSEs or A-level's. And to make matter's worse, they charged her international student fees. So a course that most of her class either got for £800 or free, my former mother-in-law paid £5,000.
So there are lots of things to consider when moving with older children. We did it, but it was not easy...on us or them. I am not trying to dissuade you. But just think that you might want to consider moving the date up...to avoid at least some of the hurdles...and getting Vicky's help to make sure you have sufficiently proven your son's dependence.
