How do you raise a polite child???

Up until tonight, I thought we were doing a pretty good job at instilling manners into our children. But the way my 4 year old acted tonight I'm beginning to second guess that previous statement. Today is my oldest child's birthday. He turned the big 0-4. We spent our entire day at home. His uncle came by late this afternoon for a visit and then one set of grandparents came out for dinner. After dinner we went back home for cupcakes. Then his other Grand-Father (and one of his Aunts) called and wanted to ride out (a 25 minute drive) to see him on his birthday. Absolutely wonderful! Well, the way my son acted was a complete embarrassment to my husband and I. I've never (to my recollection). It took us TELLING him, for him to give his Grand-Father a hug. Then, he proceeded to go upstairs. Like nobody was there. My husband got him back down stairs, after a slight meltdown. His Grand-Father tried talking to him and he wouldn't acknowledge him. Then, when they got ready to go, because who would blame them?, I wouldn't have wanted to stay either, we had to yet again, "force" him to tell them buy. I'm still so upset, I don't know what to do. He LOVES, absolutely LOVES, his Grand-Father (and Aunt) and his entire family. But tonight he was acting like a spoiled brat. And spoiled brats are NOT what I'm raising. After everybody left and he calmed down, because at this point he was just screaming, I sat him down and we had us a good talking to. I explained why Mommy and Daddy were so upset with him and how we felt his actions made his Grand-Father feel. After which, he called his Papa and apologized. But what made him act that way??? He had gotten a good nap this afternoon and like I said, we had spent the whole day at home. So it wasn't like he had gotten over stimulated. But whatever it was...I better not ever see it again. 

~Amy