I don't agree with the previous poster in that your Jack is a Normal Grey, because he's got a strong hint of blue in his colouring. My Bertie is definitely a Normal Grey; I bought him as such from the secretary of the London & Southern Counties Budgerigar Society the year before last, and there's no hint of blue in Bertie's feathers!
Perhaps Jack is a Slate Blue in colouring, then?
I've attatched a photo of Bertie taken the day I brought him home, aged 7 weeks (only left his Mum the week previous!) and you can see there's definitely no shades of blue in the feathers of this Normal Grey.