As your son is over 18, you will struggle to bring him over as your dependent unless you can show that he is living in the most exceptional and compassionate circumstances. As he is a qualified nurse and is working albeit voluntarily, the Home Office are likely to argue that he has the potential capacity to work and earn his own money even if he is not currently doing so. To make a dependent application you will have to pay a fee and if you application is refused (which it probably will be) you will lose your fee. Keep in mind that the fees increase every April and this April the dependent family member fee is due to triple to approx £1600 so if you are thinking of making the application for him and risking the fact that it will be refused, I would do it before April.
A safer option would probably be for him to apply for some nursing jobs in the UK and come as a Tier 2 worker (the old work permit). Alternatively depending on his qualifications, he could see whether he meets the rules for Tier 1 (old highly skilled migrants programme).
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