Your boyfriend has something known as a paraphilia which is a form of mental or emotional illness. Technically, the name for what he has is coprophilia, although it is also known as scatophilia (the feces being referred to as "scat") or fecophilia. Although such behaviors (paraphilias) are fairly rare, they can be present with a sexual addiction. I would suggest he get into therapy with someone who has been specifically trained to work with sex addicts, a Certified Sexual Addiction Therapist (C.S.A.T.). You can find lists of such individuals on either of two (2) websites:

Sex Help
www.sexhelp.com/

Society for the Advancement of Sexual Health
www.sash.net/

Now that you have a name for his disorder, you can go online and see what you can learn about it. I hope something here is of some help to you and your boyfriend. Good luck.

I guess I should also mention that all paraphilias, not just the one your boyfriend happens to have, are not well understood by the therapeutic community. They seem to serve no purpose of any kind, but for those who have them, they are often the only way in which they can become sexually aroused. A paraphilia has absolutely nothing to do with the individual's partner, but is rather all about the person with the disorder. It's an internal kind of thing that really makes little to no logical sense.

At one point in time, it was thought that SOME paraphilias had to do with behaviors by other mammal species. So, for example, someone who becomes aroused by licked another might be imitating the behavior of a mother dog or cat. They lick their newborns to clean them up following birth. And they lick their offspring's genitals and anus because by doing so, they get these organs to function the way they are supposed to. But in humans, these behaviors serve no practical purpose, so the reason for them remains a mystery.