Welcome to the 405th! We have the Armory section filled with Pepakura and foam templates, as well as 3D models of Halo armours and weapons. Here's a link to the Combat Evolved Mk V armour:
This is a comprehensive zip file including numerous files from multiple artists. Contents of zip-file: halo-ce-mjolnir-mkv-chest.pdo Modeler: Rhinoc halo-ce-mjolnir-mkv-forearm-right.pdo Modeler: Moz halo-ce-mjolnir-mkv-glove.pdo Modeler...
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It's difficult to say which way would be easier, it's all personal preference. Personally I prefer using foam (sometimes with 3D printed elements), plenty of people still use the Pepakura (AKA Paper craft) method and lots 3D print armour - though since you didn't mention a printer that might be off the table.
Working with foam requires cutting and gluing foam, usually some sanding too then sealing and painting. With the Pepakura method you cut out and glue the cardstock patterns, then typically coat it in resin, Bondo and sometimes fibreglass. Each way uses various tools and safety procedures (ie. ventillation and PPE) so it depends on what you and your son feel would be best for him to work with. Also, what time frame does he have to work in?
We've got some tutorials in the link below if you want a more in-depth look into these processes.
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Oh, and I like the profile picture too