The thing about booties is they're pricey and he will throw one and you'll never find it and you can't buy just one. Get a bicycle innertube and some electrical tape---cut lengths 6 to 8 inches long, slide over the big boys paws, just above dew claws, tape around his leg, then fold the other end over the top of his paw and tape again....they will wear out on asphalt, it takes a while to put them on but you'll save a bunch of cash.
http://www.petspantry.tv/acatalog/mikki-dog-boots.htmla few on here sets of 4 for £30 But have to agree he will look silly bless him and he probably wont like them either! cant you ride on grass instead?
If your dogs feet are getting sore then you are doing too much.Forget the booties and scale down the biking.How much road walking have you done with him?