I'd first investigate if there were a local instructor willing to teach a four year old. (In Edinburgh, probably not as difficult as in the States).

Then I 'd look into finding a practice chanter scaled to his fingers. (Again, probably not as difficult in Edinburgh, unless you're talking about Edinburgh, Indiana). A four year old's hands are just not big enough to cover all of the holes on a regular sized chanter, even a short one (I've met some women who have difficulty when doing some of the embellishments, like birls). Following that, earplugs - for both of you - and aspirin (or a good single malt) for you.

Best of luck to you both!