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As this is addressed to Thomas I will leave it to him to deal with
Thank you for your enquiry.
With the non-custodial sentence your son has received, he will not be eligible to apply for ILR within 24 months of the conviction. The only option available to him is to apply to extend his ancestry visa. He can apply to extend his current visa as a dependant because he was initially granted leave as a dependant under the ancestry route when he was under the age of 18 even though he is now over the age of 18. However, he must not have formed his own family unit or not leading an independent life.
The 2 options are
1. Apply to extend visa with your son as a dependant and wait for your son's window of opportunity and then both apply for ILR at the same time
2. You may apply for ILR when you are eligible and your son only extends his visa but he must meet the ancestry requirement on his own right and not as your dependant. There is a risk he may not meet this requirement. I don't know as I am unable to do any legal assessment or give legal advice on JustAnswer on whether this is a viable option.
It isn't clear from the face of rules without giving legal advice that once you have been granted ILR, your son can rely on being your dependant to extend his visa.
Without giving legal advice option 1 is probably a better option but you ought to seek legal advice on these before proceeding.
Living an independent life can be difficult to interpret. As long as you can provide sufficient evidence that he is still dependent on you. Again this needs to be assessed.
It is usually said that it is best to send/make applications together. Yes it is correct that his case is likely not to be accepted using the premium service because of his circumstances.