Of curiosity, why doesn't he have his SS paid into a UK bank account? Ours goes in promptly on the 3rd of each month, with no fees and the exchange rate is the best you can get (not a 'tourist rate'). You just need to contact the SSA and give them details of the account - AC number, Bank Name/Address, IBAN, BIC etc.
Since the US SS is only taxable here in the UK, it makes it much easier to file the UK Self Assessment as you know the exact 'value' of each payment in GB£. (And don't forget that only 90% of his SS is taxed by the UK, not 100%).
You will put the SS on the 1040, but in the taxable amount line (the only line that counts) you put $0. In order for him to claim you as an exemption, you must have a SS# or ITIN, and as nun said, you must have no US source taxable income.
Not sure what you meant by "he is under the limit for the other".