Finally...

In comments to your question, you were referred on to the UK Yankee forum which has a unique specialty in US/UK long distance relationships. I can see that you have asked the same question there. Their answers will not differ radically from mine, but you'll have the chance to ask ongoing questions in a community formed of US/UK couples.

Between now and December 2015, the Chief Inspector will be on-site at Manchester Airport at various times making a thematic inspection of the UK Border Force's processes and consistency of approach. It's reasonable to expect controls to be especially rigorous during the inspection periods, and nothing prevents you from selecting a different airport if you think that will help. The Chief has also been conducting surprise inspections at other UK airports at approximately one month intervals. None of this information is secret or otherwise restricted... It's public domain.

Between now and December 2015, the Chief Inspector will be on-site at Manchester Airport at various times making a thematic inspection of the UK Border Force's processes and consistency of approach. It's reasonable to expect controls to be especially rigorous during the inspection periods, and nothing prevents you from selecting a different airport if you think that will help. The Chief has also been conducting surprise inspections at other UK airports at approximately one month intervals. None of this information is secret or otherwise restricted... It's public domain.

Update 29 June 2015

The OP has asked the question here and has received answers from people with first-hand experience. The answers there do not change any of the answer here, but it's good to see what people with real-life experiences are saying about it. Highly recommended reading for those with an interest in the subject.

Update 26 June 2015

Between now and December 2015, the Chief Inspector will be on-site at Manchester Airport at various times making a thematic inspection of the UK Border Force's processes and consistency of approach. It's reasonable to expect controls to be especially rigorous during the inspection periods, and nothing prevents you from selecting a different airport if you think that will help. The Chief has also been conducting surprise inspections at other UK airports at approximately one month intervals. None of this information is secret or otherwise restricted... It's public domain.

The answer given by Michael Borgwardt accurately reflects the results of a cursory Google search, but fails on two points...

1. You did not ask what the visitor rules are and you are clearly aware that Americans do not need a visa to come to the UK as a visitor (your bf has visited you here previously). Instead you expressed concern about the possibility that your bf may be removed at port (i.e., refused entry) and what to do about it.
2. In comments, Michael explains that he relies upon his experience as a German national. This doesn't work because German nationals enjoy Freedom of Movement when entering the UK and thus he has never encountered the possibility of removal like American visitors do.

You are worried that your bf may be removed at port. Are your concerns real? It's a matter of opinion. The Home Office produces statistics that inform us that approximately 1 passenger out of 2,200 arriving from the US will be removed at port. Or in other terms, for every 5 arriving flights, 1 passenger will be refused entry and sent back. Some of these people have given narratives on the net and you have related these as 'horror stories', so apparently the possibility of removal is real enough for you to come here and ask about it.

You are worried that your bf may be removed at port. Are your concerns real? It's a matter of opinion. The Home Office produces statistics that inform us that approximately 1 passenger out of 2,200 arriving from the US will be removed at port. Or in other terms, for every 5 arriving flights, 1 passenger will be refused entry and sent back. Some of these people have given narratives on the net and you have related these as 'horror stories', so the possibility of removal is real enough for you to come here and ask about it. And yes, it's best to be prepared for additional scrutiny when maintaining a long distance relationship.