I wouldn't bother with an OISC immigration advisor at all.
If his application is straightforward, it is completely unnecessary to even contact an advisor, let alone pay to use one
I would only recommend an OISC advisor in the case that:
- he has serious criminal convictions
- he has a history of previous refused entries/visas for the UK
- he has worked illegally in the UK
- he has been an illegal overstayer in the UK
We can help you prepare the application and answer any and all questions you have for free here on the forum
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We have several years of experience with successful applications on the forum and personally, in the last 11 years, I have helped probably over 1,000 people to get an approved 'family of a settled person' visa.