I can't relate to my boyfriend intellectually, what should I do?

My boyfriend is one of the nicest, humblest, and least judgmental people you will ever meet. But I’ve been struggling for the longest time with his intelligence. Before I go further, I’d like to make clear that I am no stranger to stupidity or being naive. I’ve said and done my share of stupid things, but after six years of being with him it seems like he makes a lifestyle out of it. For one thing, he constantly forgets things, and not even complicated or easily forgotten things – like basic words. It’s difficult to carry on a conversation with him for over 20 minutes about, say, shopping at our local mini-mall, without him struggling for a few seconds to bring up the word “plaza” from the dark depths of his mind. Also, i’m almost certain that he’s guilty of any and every grammar or spelling mistake that peeves anyone anywhere. We just recently had a conversation because he insisted that the correct way to spell the phrase “supposed to” was “suppose to” (and likewise with “used to” > “use to”). We were also around that time talking about working out and weight loss and he was astounded when in the course of the talk he discovered that carbs and fat were not the same thing. I really can’t give a detailed description of how many ditsy things he does and says because they happen so often. I know that I’m probably being overbearing about these things; it’s more important that he’s a really good person and that I enjoy being with him. It’s just so frustrating sometimes. To say that I can’t relate to him intellectually is sort of an understatement, because he’s now a senior in college and he still seems ignorant of some of the stuff that you’d think you’d learn in middle school. I want very much to get off my high horse and get over this issue (believe me, if we are going to fight about things there are much better things that we have to fight about). Sorry for such a long winded question and any advice would be appreciated.