Why is my overweight senior dog struggling with hind leg weakness?

If this just happened, then I'd be concerned about an injury to the lower spine, or hind legs. In this case it's really best to let him rest until you are able to take him to the vet for an examination (would recommend doing this in the next day or two).

If this has been a problem that has been getting worse over a few months, then I would be more suspicious of arthritis. Being overweight is a big contributor to arthritis and acquiring injuries. Reducing his weight is very helpful for these conditions, but they often need prescription medication and therapy to help with the pain and inflammation. For this reason, it's still best to have him evaluated by a veterinarian.

It's good that you are concerned about him getting overweight, because he does appear to be somewhat chunky. In order to get started with a weight loss plan it's good to score his current body condition. Below is a link that will help you do this.

https://www.wsava.org/sites/default/files/Body%20condition%20score%20chart%20dogs.pdf

After that, you can use the healthy weight calculator at the following link: https://www.petcoach.co/healthy-weight-calculator/

There are some products in the pet store that are actually formulated for weight loss and these would be the best ones to feed. I know Hills, Purina and Royal Canin all have good products.

The best thing to check on the bag is the AAFCO statement, which is just below the guaranteed analysis. This will tell if the food has actually demonstrated what it claims. I've included a link to an article below that explains all of this. Make sure you feed the food according to the bag's instructions and try to cut out all treats and table scraps.

https://www.petcoach.co/article/what-to-look-for-when-choosing-your-pet-s-food/

The hardest thing for pet owners is that they feel like they are starving their dog. Trust me, you are not starving them, despite how bad they may make you feel. If your dog likes any vegetables like celery, carrots or green beans, then these are great, low calorie options to give as treats that can make you and your dog feel a bit better about the diet.

In the end, know that a skinnier dog will be healthier and live longer, so it's well worth it.

I hope that helps and you are able to get a good plan going for Polo. I've given one more link to an article with additional information below.

https://www.petcoach.co/article/5-ways-to-help-your-dog-lose-weight/