
Have you had his thyroid checked? If he's always this active and the weight is fairly recent then I'd be checking that, especially if you're really struggling to get it off him. Dogs don't always display other symptoms such as hair loss etc (and some show next to nothing or indeed nothing at all - I know of one dobe who tipped his owner off by starting to just clip the poles on the agility jumps, nothing else changed).
Other things that have worked for me have been mixing in a little bran (if he's not wheat intolerant, of course) to bulk it out, or soaking the kibble overnight/through the day so it swells and fills him up.
The one thing that's worked the best for my boy (who has been overweight/up to obese for the last 4 years following an injury) is feeding Skinners salmon and rice - it's the only food that's gotten weight off him, although even with that I have to be careful. not saying it'd work for your fellow, just that a change of food might be helpful :-)