Lots of great advice on here already... I'd definetly add fruit and vegetables - I normally bring an apple (Fuji apples are amazing), an orange (they have those smaller ones that are seedless and peel easy), and baby carrots. The baby carrots are cheap, taste good, and may help keep his eyesight keen for the welding. I also throw in one of those fruit gummy packs - like gummy bears for kids - you can find them with 100% vitamin C and a sugar pick-me-up for the second half of the day has never steered me wrong.
Hydration will be very important. I like the suggestion of iced tea with a smidge of salt to create home made gatorade.
Tough to beat the old tried-and-true peanut butter jelly sandwhich. Protein (peanut butter) + sugar (jelly) + carbs and fiber (good quality bread.) I'd throw at least two of 'em in the igloo and see if two is enough to sustain his energy level.
A good place for fire resistant clothing is
WorkingPerson.com . Don't let him use clothes that are synthetic in any way, as they will melt and stick to the skin if a good spark hits em. Cotton is good - this place carries fire protective stuff for welders, military, even firefighters. They also give ya a discount for military service and free shipping. I don't work for that company or anything - just been happily buying gear for several years and have found it all satisfactory, good customer service too.
He's a lucky guy to have someone that cares enough to look into nutritional information and help keep him fed. Plenty of young men survive off of frozen pizza (I know I did!) Good on you, and good luck to both of you.