6lbs. at 3-4 months old would be the growth rate of a commercial typerabbit maturing at a weight of 10-12 pounds. In the US, suchbreeds would be New Zealand, Palomino, Satin, Californian,etc. Its difficult from your photo to get an idea of bodytype, so its hard to make a more accurate guess at his breed.We have a similar blue breed in the US called an "American" and it hasa semi-arch body type (long over the shoulders).
As others have suggested, I don't think he'll mature into one of the Giant breeds such as Flemish.
And no, the "tiger rabbit" is not real -- it is a computer generated graphic.
You may want tocheck out the BRC website
http://www.thebrc.org/andlook under breed standards. Your rabbit does resemble theBlue Vienna that we once had in the US. I'm not sure howcommon they are in the UK.
Pam