I thank you for your enquiry and your patience.
1. He can apply for the spouse visa in America, he however would need to wait in America for the permit before travelling to the UK. He cannot apply and then travel to the UK while waiting on the decision to be processed unfortunately.
2. He cannot apply for the spouse visa from the UK. He would have to apply from outside the UK.
3. He could not switch from a visitors visa to a spouse visa in the UK, so unfortunately I would not recommend this route.
4. Ordinarily, he could have applied for the visa to be processed using the premium service. This would allow for his visa to be processed within 4 weeks. This facility, as far as I am aware was suspended during the pandemic. He could check when he is processing his online application whether the premium service has resumed. If the premium service has not resumed, then unfortunately, he would have to submit his application through the usual route.
Can I clarify anything for you?
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.