Ah, that's a difficult situation, isn't it?
Can you bring a photo of your fiancÚ? And then say somethig along the lines of:
Unlce Bill, did you hear that Adam and I are engaged? He did so want to some and meet you, after I told him you were such a nice person, but. alas, he is a musician and you know the kinds of hours they keep--he's working this weekend and couldn't come with me, as much as he wanted to. This is what he looks like. he is [your description of a wonderful person] and I do hope we can arange for you to meet him.
Of course, you'd do this in your own words, but the idea is that your fiancÚ wants so much to meet your uncle and was devastated that he couldn't come with you and that you are optimisitc that Uncle and FiancÚ will be able to meet one of these days. You don't have to say anything silly like "I'm sure you'll be well and able to come to our wedding." Just say enough so that you (and Uncle) have the comfort of knowing that Uncle knows you have found Mr. Right and are very happy.