Cotton welding clothing is flameproofed. It works, but leather works better, IME. OTOH, TIG-master Ernie uses cotton exclusively (other than gloves), as far as I recall from a previous discussion. Depends what you're working on, and what that exposes you to - arc light is one thing, molten slag is another, and molten steel is a step up from slag.
If your son is getting burned through leathers, he needs heavier leathers, or he needs to reposition himself with respect to the spew, or he needs additional sheilds in the affected area if he's got no option but to be under the spew. Kevlar sleeves offer some heat protection in an easy-to-wear form, various specialized multilayer pads offer a good deal more protection, at the cost of being bulier and less comfortable.
Still getting burned through leather? He is either positioning himself right under the puddle, or using some very big rod. Positioning ones self is a very important thing in welding for reasons of visibility, slag, and so you don't run out of room or handcuff yourself. And "sparkes" shouldn't "just stick on the leather and burn." What kind of leather is it? Does it have something on it that shouldn't be on it?
Second the bit about being positioned right. When your elbow is bent, the leather bunches up on the inside. If you are welding overhead, slag can catch in there. Solution is to smooth it out / position better / wear more clothes / deal.
Makes one long for the good old days when you could get fabrics woven out of asbestos... The point about "which" leather is well taken. Not all leathers are created equal, the thicker and tougher the better.-Jitney