Hi there
Tempting though it is, I wouldn't suddenly feed him huge amounts because he looks like he needs it - especially if he's young and still growing since that could cause problems.
Dog food makers are notorious for writing on the packet that your dog needs bucketloads - that's good for their business! Whatever you choose, I'd go for a good quality make which isn't bulked out with cheap cereal products and doesn't contain additives such as colourings.
I'd have a word with your vet to be on the safe side. But for on the spot free advice, you might also like to post on another forum where a canine nutritionist I know - and other knowledgeable people - will most likely be able to help.
It is at
http://www.ttlntl.co.uk/forum/ and you would need to post under the 'health and diet' section.
Of course, other people here may have had similar experiences with their rescues and be able to tell you what they did. :D
Good luck!