It depends on what sort of rash it is, if it's raised bumps then it's most likely an allergic reaction of some sort, and it also depends on where on his face it is - if it's something specific like bright red patches on the cheeks then it might be slapcheek, although he's a bit young for it.

I know some kids develop blotching on the face from too much fruit, but as you say, it could be irritation from wool fibres or something like that.

Sorry to be so non-specific, but without seeing the rash, it could be anything. Most likely nothing to worry about, though, just a minor irritation.

Kerrie