Personally, I do not see an issue in people of different faiths dating. There certainly is not any direct guidance as such from the sikh rehat or the scriptures. Often with Sikhism punjabi culture is seen as interwined with the religion --of course this is completely incorrect. What your boyfriend is describing is a sort of racism that is unfortunately rampant among punjabis
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-youth/3091-can-sikhs-go-out-people-not.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=3091

On the other hand, being a minority culture, there is always a tendency to want to protect it. Such protectionism is common among many minority cultural groups. In my humble opinion, this does not excuse it but I hope that it at least serves as an explanation. In most cases there will be an immense social stigma for the child not to date outside of the community (or to date at all). The fact that traditionally parents chose mates for their children is also partly to blame for the bias against dating. This is of course terribly old fashioned (it is very rare for parents to choose their child's mate these days) however, again, I hope that it serves as a sort of explanation for where he may be coming from.

take care,

Ravi Singh