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.
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...
Macdonald's Immigration Law & Practice 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
For further research, see Exploring the decision making of Immigration Officers: a research study examining non-EEA passenger stops and refusals at UK ports
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.
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.