My grandson is a long-legged, skinny 5yo, too. His mom finds elastic-waist pants in his size (Old Navy, I think). He HATES non-elastic waists that he has to hike up all the time, and I sure don't blame him.
Comfort, utility, protection against the elements, and modesty are really the most important qualities in clothing. Beyond that, fashion is a matter of taste and not even important to many kids. When I look at the clothing choices of many of the children of my acquaintances, I am charmed by all the ill-fitting, old, wildly mismatched, and obviously loved outfits they wear – even portions of old dress-up costumes, like half-shredded tutus in a little girl I know. All the way through grade school, for many of them.
Lots of kids, additionally, have sensory-integration issues to some degree; seams or textures or tags really bug and distract them. I've always been like that, and can scarcely endure anything rough, especially around my neck. I've been known to wear jammies, shirts and socks inside out to escape the seams.
As far as setting this up as an inevitable battle, how about backing away from that for a month and checking out the compassionate and mutually-respectful techniques employed in How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. Your son(s) can be a working part of just about any solution you may need in coming years. These authors show you how to do that in ways that will meet both your needs and your sons, with less stress. This is a lovely book – I use it with my grandson all the time, and we work through inevitable behavioral problems creatively and effectively.