The safest, legal way is going to be a fiance visa. I know it's hard, believe me, but these rules were created without what you want to do in mind. He can try BUNAC immediately after he gets out of school. That allows him to work in the UK for 6 months. Other than that, a student visa seems to be the best option. He can spend 6 months here, but he will be grilled,and possibly bounced (not allowed entry into the UK) if he says he is coming here to stay with his gf and he does not show proof that there is something waiting for him back in the US, plus he needs to show he can support himself for 6 months while here without working. He can leave and try to re-enter to get another stamp for 6 months, but the more he does this, the more likely he is to be flagged in the system and then get caught abusing the system and it can be seen as deception as he's not on holiday but actually living in the UK. Also, bear in mind the UK is looking to reduce the number of months a US citizen on a US passport with a tourist visa can legally stay in the UK to 90 days coming more in line with the US system. Also, they are cracking down on those who overstay with lying to the Immigration officers or committing any kind of deception leading to him being barred for a certain amount of time. The advocacy group is looking into the implications of this now, and will probably put information about this up later.