As a US citizen, he does not need to apply for a visitor visa before travelling - instead he will be issued it as a stamp in his passport when he arrives in the U.K.
However, the longer he tries to stay for, the more risk there is of the immigration officer being suspicious that he is not a genuine visitor. So while technically a visitor can stay for 6 months, immigration don’t have to let him in for that long if they think he is an overstay risk.
There’s no official rule about time spent outside the U.K. between visits, but if he’s spending more time in the U.K. than in the US and it appears as though he is using the visitor visa to ‘live’ in the U.K., he can be refused entry and sent straight back to the US. A general rule of thumb is to spend at least as much time outside the U.K. as he
For each visit, he would need to show strong ties to the US to prove he will leave and not be tempted to stay in the U.K. illegally:
- a return ticket
- enough money to support himself without working for the length of his visit
- a job to return to in the US on a certain date (a letter from his employer)
- a home he is maintaining in the US (valid rental agreement or mortgage evidence)
- any other ties like family he cares for, commitments, appointments, events etc.
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