Your son should be fine.  
 
In the demographics of Salt Lake County (they county where the LDS church is based) only about 50% of the people actually identify themselves as LDS. In Cedar City where SUU is located the number is going to be less. That means that the majority of those who will be teaching him and who are going to school with him are not going to be LDS.  
 
Most of the information out there about how Mormons view others is highly misleading. The ones that impress me the most are those who claim that they were once Mormon and are no longer. They sometimes say the most random things that are completely inaccurate. Mormons are taught to be non-judgmental and loving to every one. Most members that your son meets should be a good example of this. 
 
I have heard from many non-members who have moved to Utah that they had heard crazy things about Utah, but once they moved here they realized it wasn't so weird after all. If he lives in a high LDS area he may have a bishop show up at his door to see if he wants to be invited to church events. Our bishop checks in on the non-members once in a while just because we have an emergency plan in our ward that includes helping the non-members too, and he likes to make sure that he is aware of what is going on at their homes. It's a non-threatening, non-proselyting type visit.  
 
As far as the young woman that he is dating, there should be no issues. There are lots of interracial couples in the LDS church. My ward alone has a couple that is just as you described. We also have a Caucasian guy who is married to an Argentinean, and even our previous bishop is married to a Korean woman. (The bishops are only called for about 5 years in our church)

First, I agree 100% with all of Dromidaon's comments 
 
Second, I am LDS, caucasian, and married to a guy from Chile (Latino). Interracial marriage is very common in the LDS church, perhaps due to the missionary culture in which we live. Thousands of young men and women serve 2-year church missions all over the world and often develop a great love for the people they serve, no matter their skin color or religious denomination.  
 
I think the missionary experience makes Mormons more open to the concept of interracial dating and marriage, not less. 
 
Also, MSNBC aired a special in August or September of this year (2012) that highlighted an interracial Mormon family with several children. He was black, she was white. You can probably view the special or parts of it online. Just do a search for Brian Williams Rock Center Mormons in America. 
 
Best wishes to you and your son.