My son is not fat, but he has a full bottom and big upper thighs. The Next 'Plus' range fits him perfectly, but they only do school trousers in this range. He fits into his own age tracksuits and elasticated trousers so his leg length is pretty average, but his 'girth' lets him down 
! As he's now getting more fashion conscious he wants some skinny jeans, but I have never found any that go anywhere near him - I think we'd need to buy age 10/12 to get them done up, but they'd be far too long and modern jeans lose their shape if turned up! I look forward to some good advice from you experienced mums out there. Thanks very much in advance.