Sizing up a knitting pattern

[Ladybee]

Hi,
I'm still a novice knitter but have completed several things for my DS and now DH is getting a little jealous He's asked me to make something for him, the problem is he's a big guy. I've found a relatively simple cardigan pattern but it only goes to XL and I don't think the 'To Fit' measurements are big enough for him.
How difficult is it to size up a pattern? Or, is there anyone out there who would be willing to help me out on this? It's this pattern - Mens V Neck Cardigan in Rowan DK Pure Wool Family Collection. I'm a bit annoyed as the back of the book says Rowan patterns are available up to XXL, but as far as I can tell, none of them are actually shown with XXL instructions. I've bought extra yarn etc...can anyone help?

can't you work out the stitch difference between the small and large and then apply that to the difference you would have between a large and your xxl??

all the shaping would be the same, you'd just have the extra stitches to remember.