1. Dear Alla, there is no chance your son's visa will be cancelled just because your husband's visa might be cancelled. Essentially, it is you having a Tier 2 visa which gives your son the right to have a dependent visa himself. So long as you continue to have a Tier 2 visa your son qualifies to have a dependent visa and nothing your husband does will affect this. Your son's visa is only dependent upon you, not upon your husband. So, you don't have anything to worry about here. There is no rule that both parents must be in the UK for your son to have a visa. So long as you are your son's guardian, he will qualify for a visa dependent upon your tier 2 visa.
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3. Dear Alla, the issue of Family separation is more important when it comes to obtaining Indefinite Leave to Remain. In point of practice, your son won't get ILR if he only has one parent living in the UK. In order for him to obtain ILR, he would need both parents living in the UK. However, once you have got a visa, you will continue to have it. For example, if your husband wanted to remain in the UK after you divorced him, there is provision for him to retain his visa so long as he has at least 2.5 years spent in the UK. So, the advice is different for your son once you have already got a visa for him. Whilst the Home office is not desirous of family separation when first issuing a visa to a person, once they have a visa, then they are in a stronger position. The presumption then is that they can retain this visa. Accordingly, so long as your son is resident with you, then he can retain his visa. He won't be expelled from the UK merely because you divorce your husband.
4. That is an accurate summary of the position. Essentially, if your son wants ILR and naturalisation, he will have to wait until he is 18 and able to apply on his own account as he won't be able to apply himself whilst a minor unless you remarry. However, he will get an extension of his visa from 2.5 to 5 years, so long as you, his guardian are resident in the UK, even if you are divorced.