@davy Walk him as much as you want. The only concerns would be:
Is the pavement hot/burning him?
Is there salt hurting his feet?
Is he having trouble breathing/overheated?
Is he on cage rest for a medical condition?
If not, walk on. If you need to adjust his calorie intake, you adjust it, but you don’t cut
down on walks, just increase food (if necessary). After his neuter, he may pork up a bit on his own. A lot of people think their dogs are underweight because we are used to fat dogs. Feeling ribs isn’t a problem. If you are seeing all of them plus spine? Yeah, too thin. But a lot depends on how a dog is naturally shaped. Breeds like Boxers, you’re doing to see their rib cage unless they are so thickened in the waist that they don’t have a distinct tuck. Confer with your vet about body condition/caloric needs. But your dog is very lucky to have an owner who enjoys being outside with him so much.
Source: vet med for 12 years