Hi, I am Dr. Peter and would like to help.
1- What do you feed him?
2- Any table food?
You are right to believe this could be related to having an under active thyroid (hypothyroid), but, in a dog this young it is more likely to be nutritional. When dealing with biological systems however we often see exceptions. I would recommend to have your veterinarian take a simple blood test and check his thyroid levels. If they are normal, than it is purely nutritional. When addressing weight issues in dogs one can either feed him less of what you currently are feeding or change him to light formula and feed according to the label recommendation for his target weight. Example. If you want him to be 10 pounds you feed him for a 10 pound dog. The best way to target is weight is by his body condition. If you follow the link below it will teach you to evaluate his ideal weight. I would recommend to eliminate all chicken, sausage, etc. Put him on a light formula and feed according to their recommendations. In addition, increase his activity level to burn excess fat; take him for walks and gradually increase the distance.
If you have additional questions I will be glad to answer them.
Yes, animals that are neutered can gain more weight easier as they do not have the testosterone available. However, this does not mean that a dog will be overweight if neutered. It means that with the same intake he can gain more weight than a dog that is not neutered. Making dietary changes can maintain a normal weight.