I guess, just like for humans, it's eat less, move more.
When you say two really good walks, how far do you mean? Could you increase them? Can he be off lead for longer? Does he currently tear around much when off lead, or does he walk sedately to heel? If the latter, worth looking at what you can do to encourage him to explore more.
Re the jogging, I am fully prepared to stand corrected but I thought it was not advisable to go running with your dog? Because the dog may need to stop and rest when you don't? I imagine there might be a particular risk for a dog who currently needs to build his fitness (and especially in the hot weather), but I gather it's a bit risky fir any dog. This is what dh assures me - hence why he both runs miles himself a day, and also separately walks the dog for miles twice a day, rather than combining the two (time consuming) activities. He could be wrong, but worth checking out.
I'd just like to say, at the risk of sounding patronising, good on you for confronting your dog's weight problem, and doing something about it. Round here (in the countryside!) we are surrounded by overweight dogs, and the owners all either in denial, or don't care.
My own dog looks too thin to me, tbh, but we're assured by our vets that she's just the right weight for her breed and size. She's nine, but so playful and active she's often mistaken for a puppy! So it's well worth the effort of getting and keeping the weight off, I would say. Good luck!