Step 1:
Call in at your nearest branch of Pets at Home. Store finder here:
http://www.petsathome.com/storelocator?_s_icmp =3l39TRr1
Step 2:
Purchase some Simple Solution Stain and Odour Remover:
http://www.simplesolution.com/products_uk/so_r emover.html
Step 3:
Really drench the affected areas of the seat with the stuff. (You're not just trying to mask the smell. You need to 'kill' it completely. The odour remover can't do that unless it comes into
direct contact with every bit of the sour milk).
Step 4:
Park the car somewhere secure, so you can leave the windows open while the seat dries out. (That's just to help it dry quicker. The solution isn't hazardous in any way).
Step 5:
Smile sweetly at your son ;-)
Chris