In light of the fact that this question has been asked of me at least 10 times in the last month, I've decided, in the middle of the night, while I was pondering exactly how this came to pass, that I'd share my thoughts on the matter.
So, in light of parenting advice (which means you take what makes sense to you and make it work for your situation) here goes.
First and foremost... a LONG time ago, when this child was yet a baby, then as a toddler, he had no choice but to attend functions, related to his older siblings being talented, busy folks.
I've decided that the first step to 'comfort in dress clothes' is that he was always 'along for the ride' and always dressed appropriately. In other words, from way before he can remember, he didn't know there was any other choice. Frankly, with me as his mom, there wasn't any other choice... We simply dressed him, like you'd dress any little southern boy for dressy occasions and tossed him into the car seat and carried him along.
His siblings before him were also appropriately dressed and carried to many 'adult functions' where they too, had no choice in the matter.
We had neither the option to leave them with a sitter, nor the desire. Not only does this produce kids that willingly wear dress clothes, but also produces kids who grow to appreciate many of the fine arts and grow to tolerate many of the things that we as adults must attend.
Had I not carted the kids along, during those critical 'small children' years, then I'd have had to stay home and miss way too much. So, for my need for intellectual stimulation and my belief that they learned much that they needed for later, they got to do what I wanted to do.
So, from a young age, the little man wore his dress clothes and sat through hundreds, if not thousands, of church functions, concerts, ballet performances, recitals and family functions in general.
Now, I am mindful of the fact that he sat through roughly three times as much as say, his oldest sibling.
I think that plays a part.
Part two involved the clothing itself. You simply have to get it right. It's not negotiable that it be a proper fit, adequate for the weather/event, that it be in good repair and here comes the biggie...
... that the kid like it. This means that you have to spring for GOOD clothing. A good brand will fit and feel 'better', often, and it will wear better and longer and the kid will simply be far more inclined to wear it because people say things like, "My goodness, don't you look handsome tonight!" This means that you cannot wait until the last minute to throw together a dress outfit. This means that you need, in the closet, two choices, ready to go. In the case of boys, this means a navy sports jacket/khaki pants combination and by the time they're teens, a good fitting suit. You need solidly 4 dress shirts, including two white ones and a selection of ties. NOT dad's ties... not grandpa's ties... not kiddie clip on ties... nice quality ties. Dress shoes are also necessary...
While I do believe in doing things yourself, and I know how to do it, this is where I part ways with frugal beliefs and I take the dress clothing to the cleaners. Crisply starched shirts and khakis can't be substituted for 'let me see if I can get this ironed well enough for a concert?" sort of wear.
Yes, boys grow... fiercely at times... so this means that by the time I got to the second boy, he wore some of his brother's former dress clothes and I've needed to purchase some just for him. Luckily for the budget, the boys have similar tall, slender frames.
Anyway, before any dressy event, you have to try the dress clothes on for fit.
And making sure that the garments are good quality and stylish is important if you want the kid to LIKE to do this.
Third... as a parent, you must NEVER say things like "Don't do that... you'll get dirty!" "Be careful... you'll scuff your shoes"... You must actively encourage them to behave normally in their dress clothing. This means that when they're small, you're gonna have to go back into the closet for the 'back up' dress clothes sometimes. This means that your heart will lurch when you see an expensive dress up near the top of a tree or you come out of the house and see a little dressy pair of starched khakis in mid-flight over a mud puddle.
You just tell yourself to calm down... it's clothing, after all, nothing more.
I'm convinced that half of the battle at least is this. If they're allowed to be 'normal' children in their dress clothes, then they will not mind wearing them. They will learn to 'look comfortable' in the moment, regardless of the clothes.
And finally, you yourself, must make it a priority. You have to dress and go. You have to do the 'sit and cheer' things, regardless of whether you want to each and every time or not. As much or more than anything else, you must be their biggest fan. For most of us parents, this is an easy thing to do. But from looking around, it's getting harder and harder for parents to make it a priority. Yes, you have to alter your work schedule, yes, you have to plan ahead, but YOUR willingness to be present and accounted for makes that kid more comfortable in his or her own skin. I promise. The confidence and self assurance that any child has begins in knowing where they stand in their own family... that they come in high on the list of priorities... that they matter enough.
As I have said more than a few times here... the days are short that you'll need to do this... make sure that they know by your presence. There's plenty of time to do the other stuff when they're gone.