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][1] 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][2] 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.
Also, you and your partner have correctly determined that there is nothing wrong with maintaining a long distance US/UK relationship so your partner has decided to be honest about it. Like "What is the purpose of your visit?" answer: "*I am maintaining a relationship with my gf that has been ongoing for a year and this is my second visit in that role*". A direct reply like that signals candor and they like that because it means they can avoid the whole cat-and-mouse game about who your 'friends' are. The candid reply also tells the Immigration Officer that your bf's accommodation arrangements are sorted so they can continue on to the rest of the interview. In their minds, claiming to be visiting friends when there is an intimate relationship at hand is lying, they don't like it.
The IO may go on to query your bf's financial standing and he should bring along a some recent bank statements to help demonstrate that. He may also **phone you** in the arrivals area and ask YOU a few questions, like is he your bf? Is he your fiance? What does he do? (a trick question, be careful). Is he looking forward to a new life in the UK? (another trick question, caution). Again, the honest approach will eliminate the need to 'make your stories jive' and allow you to answer with confidence. They like that also. Some of the saddest removals come from wholly innocent people trying to contrive something and then getting caught in a discrepancy where had they been honest the partner would have sailed through.
If he qualifies, he will be stamped in, usually for 6 months. If he does not qualify, he will be removed.
**Nailing it down beforehand...**
If you are really *really* paranoid you can advise your bf to get an entry clearance. It costs GBP 83 and Americans do not need to get one, but can get one if they want to. When an American gets an entry clearance as a visitor, the landing interview is reduced to a brief formality to assure that their passport isn't forged etc.
Gina Clayton mentions this in her book [Textbook on Immigration and Asylum Law][3]...
![enter image description here][4]
[Macdonald's Immigration Law & Practice][5] also mentions it...
> ...They [general grounds for refusal] do not apply in cases where the
> person seeking entry already has entry clearance because in that
> situation the grounds for refusing leave to enter are much more
> restricted...
Section 3.54, pp 31
**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][6]. 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.
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For further research, see [Exploring the decision making of Immigration Officers:
a research study examining non-EEA passenger stops and refusals at UK ports][7]
[1]: http://eur-lex.europa.eu/browse/summaries.html
[2]: https://www.gov.uk/government/collections/immigration-statistics-quarterly-release
[3]: http://www.amazon.co.uk/Textbook-Immigration-Asylum-Gina-Clayton/dp/0198701896/ref=sr_1_1?ie=UTF8&qid=1435271156&sr=8-1&keywords=gina%20clayton
[4]: https://i.sstatic.net/GdA31.jpg
[5]: http://www.amazon.com/Macdonalds-Immigration-Law-Practice-Edition/dp/1405754753
[6]: http://talk.uk-yankee.com/index.php?topic=85525.0
[7]: http://www.statewatch.org/news/2007/jan/uk-ho-immig-decision-making-study.pdf