Weight alone isn't a real guide to whether or not a dog is obese

A boxer is
supposed to be 23-25 inches tall and weigh around 65-70lb, but if you happen to have an individual that is substantially taller than the breed standard then that individual would naturally weigh quite a lot more at a "correct" weight.
As mentioned, you should be able to see the last two or three ribs. Note that that is when the dog is just standing normally. If you can only see them when the dog stretches, or have to poke a bit to feel them, then the dog is overweight. If you *never* see them, and can't feel them even with pressure, then that's an obese dog that is being done no favours by an owner that refuses to recognise it.
Excess weight is something that causes a lot of health problems, most especially as a dog starts to age - so if your vet feels he is overweight, then addressing that is very very much in the dog's interests.
Anyway - it is great that you're taking that advice seriously, and starting a weight reduction plan for your guy. Between increased exercise and perhaps a few less calories, I'm sure you'll have him in shape quickly. And it is so much better for his health. Just like people, dogs lose years of life to obesity and it really is in our hands. LOL - you'd be surprised how many owners ignore their vet's advice year after year and insist that their dog is just "solid", whilst simultaneously dreading the lecture they know they'll get when the dog has gained yet more weight... :rolleyes: