Hi,
I purchase a lot of western wear. High Noon like most other western wear runs slightly smaller in size, it is considered a slimmer western cut fit. It's best to size up one size when purchasing western style clothing/shirts. Shirts purchase at Walmart, target, kohl's, etc....tend to be more roomier and wider and considered to be a comfort fit. If the Walmart shirts in XL are roomy on him than you may be ok with an XL in this shirt. If the XL shirts are somewhat snug, then you will want to size up to 2XL when purchasing western clothing, I hope this helps
Bob