Your son is wrong...for auto body work it matters a lot.

Solid wire with gas MIG all the way...

Flux core wire works very poorly for thin gauge sheetmetal...expect frequent blow-outs, excess spatter and slag to grind off, welds full of pinholes, etc.

A properly set up gas MIG with sizzle like bacon and give smooth, clean welds with little spatter and minimal blow-outs.