I work with a guy that said to me that if they ask me "whom i am staying with" and I answered an 'online boyfriend..'. He said that they would send me on the next plane home as IO's don't like the online gf/bf thing and get suspicious!
If you say 'online boyfriend', then they may question you to make sure that you aren't planning to try to stay in the UK illegally and that you have a reason to return to the US at the end of your trip. If you can prove that you are just a visitor and are not trying to use the visitor visa to live in the UK illegally, then you shouldn't have an issue. If you do have a problem, then the worst that can happen is that you are refused entry to the UK and will need a visa to return.
However, if you say 'friend', but then they question you even further and it comes out that he is actually a boyfriend, then you can be accused of lying to UK immigration, refused entry from the UK and banned from the UK for 10 years!
So, weighing it up, there is a risk of refused entry whatever you say, but saying 'friend' when he's actually a boyfriend has much more serious repercussions than just saying 'boyfriend' to start with.
The best thing to do is be honest - if you're not sure what to call him, then maybe just say so to the immigration officer... i.e. 'at the moment he's just a friend, but we're going to see how well we get on in person'.