Also he is 15 kg and he is a cavoodle. He's had chocolate before but reacted much differently
If he is eating, peeing, and pooping, he is probably not having any chocolate issues. After about age 6 or 7, dogs slow down. If the dog is overweight, it can hurt to walk, especially for older dogs. Dogs can also get arthritis.
Check his toenails for a broken nail. Check his pads for injuries. Run your hand up and down his legs. If he tolerates that, try a little massage action.
Honestly, it doesn't sound like he is having ill effects from eating the chocolate. If it were that, you probably would have known within a few hours, with possible diarrhea, vomiting or general signs of illness.
Some dogs can get away with it. Some can't. Obviously chocolate is a bad idea, but it sounds as if your dog will be ok.
100g of milk chocolate chips are not too bad. The worst for dogs is dark and baking chocolate.
But please do contact your vet and have a word about it if you are worried about any possible damage.
I had a Jack Russell once who sometimes snaffled chocolate. I never gave it to him, but sometimes he would find it here or there in the street, and eat it so fast before I had chance to stop him! He never once showed any ill effects! And lived to be over 14 with no liver or kidney issues or anything else, passing away from prostate cancer in the end.
But I had another dog who daren't even eat a tiny bit, or she would be violently sick and have diarrhea!
I am sure the limping and stiffness has nothing at all to do with eating chocolate. You should definitely get that checked with the vet as if it's arthritis he could be in some discomfort.
Theobromine toxicity usually causes dogs to be hyper, they pant, they develop neurological symptoms. If it was milk chocolate, it is considerably less toxic than dark chocolate, with baker's chocolate being highly toxic. If your dog had theobromine toxicity symptoms would have manifested within a couple of hours.
With regard to the limp, it's not likely that this is related to the chocolate. It could, however, explain her slowing down and acting different. A trip to the vet, unless there is something glaringly obvious (like a sliced pad or broken toenail) will be your best way of getting to the source of the limp. Please post back and let us know how your pup is doing.