How should I build an air line?

Your nephew is correct. DO NOT use PVC for compressed air at any pressure. See this and this.

Use either steel (iron) or copper. I prefer copper. If you select copper, use the 95/5 tin/lead solder rather than 50/50 because it provides a higher pressure/temperature rating. The weak link in a soldered copper piping system is the solder joints, not the tubing itself. I prefer type L rather than type M because it provides a little better protection against physical abuse. Of course the best from a physical abuse standpoint is steel with all socket-welded joints but that is overkill for most home shops.

Rod

As stated below, DO NOT use PVC for the system.

Shut off valves should be installed at each end of the run. Also install a valve at the end of the drip leg for easy blow-down and drainage of the line. Also install a regulator and coalescing filter at the end of the run. By this time, the compressed air has cooled somewhat allowing moisture to drop out of the airstream more readily than having a filter at the source. The regulator should be equipped with a pressure gauge, too.

I don't have this but a neighbor does and he loves it. It's some new fancy aluminum air pipe system with fittings you just connect by hand, absolutely no tools needed. They sell it in raw pipe and then you cut to fit and buy only the fittings you need. He said it took him longer to decide where to put the pipe than it did to install it. Looks very professional. He got it at a big home center.

Either>Link HD or Menards, the only two we have.

CU, type k, makes a fine air piping system. Looks good, strong and will handle 1000psi plus depending on installation method.

I like using Harris 15% silver solder, about $22/lb, as it's viberation resistant and strong.

Clean the pipe with 80grit emery paper and the pipe fittings with a fitting wire brush. NO acid is required. Use Oxy/Acet torch with a "O" size tip with a slightly reducing flame.

Heat the joint with a medium flame until the solder will flow, then move the flame towards the rear of the joint thus sucking in the liquid solder thru out the joint. Add enough solder until a slight solder dimple forms at the bottom of the joint seam, then stop as the joint is full.

Run the main line at a 1/8" drop per 12" of run and take the "T" drops off from the TOP of the main line run with a 6" drain at the end of each vertical drop. Use a flex connection to the main line from the compressor, hydraulic hose works well.

A well designed piping system equals more usable CFM you will have by using the least amount of pipe fittings possible to get the job done.

T_Bone