Hi! Depending how old he is, try Debenhams, which have recently introduced a Jasper Conran Junior range www.debenhams.co.uk or Gap Kids, which has a big range for you to splash out on. Both store have some pretty funky options! Mr Mackey, mmkay!
Rather than spend a fortune on "designer" flares you could do what I did when I wanted some flares: Take a pair of jeans/trousers, find some material that you like (anything-almost-will work. Carefully open up the outer seam of the jeans legs to mid-way up the calf, remembering to stitch closed the top of your cuts and then add in the material you've selected. Doing it this way not only saves you a lot of money but you can customise the amount of "flare" and have a truly unique item of clothing for your son. I used purple velvetenn btw and I think they look great!!