We have no magic cures for older dogs getting slower and weaker on their legs - if we did, we would be writing from Monaco, not Hertfordshire!
However there are a few things which can help, but they are the obvious ones:
1) A slim dog does better than a plump one. If he were to lose some more weight, he would get around better. That is without doubt. It is also true that if you feed him less, he will lose more weight. He would be left feeling hungry all the time and that might seem too mean to do to him, but he would get around better. It is up to you to decide where the balance should lie. Personally I would feed a senior profile, good quality complete diet, in the quantity which produces the ideal body weight. If he is overweight at all, he is getting too much to eat. It is as simple as that, though there can be reasons to accept over-eating for emotional reasons. Up to you.
2) Regular gentle but not prolonged exercise. It is important to keep the muscles exercised and the joints supple. Frequent short, interesting walks are much better than occasional long walks.
3) Anti-inflammatory medication. Although you might feel muscular weakness is the main problem, this usually goes hand in hand with sore joints which are wearing out with age. Then the dog puts reduced stress on the joints because they are a bit uncomfortable, and the muscles do weaken. It would be remarkable if there were no joint trouble at all in an older dog, and it is unlikely. The best test is to try a course of anti-inflammatory medication (vets tend to use Rimadyl, Metacam or Previcox usually). If you try this for 10 days or so, you might notice a marked improvement in your dog's mobility. If so, then joint problems are actually contributing to the situation you notice, and there is something you can do about it. Most older dogs of the larger breeds do benefit from these types of medication in fact.
4) Nutritional supplements. There are loads of these which contain glucosamine, chondroitin and various other, possibly useful additives. There is a lot of proper scientific research to show that most dogs will show a slight improvement in mobility if given these products for at least a few weeks, ideally on-going. Synoquin is the original, most properly evaluated one we believe. They definitely do work to some extent, but are a bit limited realistically.
5) Acupuncture. Not sure what to think about this. It probably does work very well for a few animals, and hardly at all for many. Not really properly understood. Unlikely to do any harm.
6) Any of the other multitude of "alternative" types of medication or treatment (homeopathy, crystals etc). Unlikely any of them do anything at all in my view, but clearly many people believe in them.
Personally I would advise gradually reducing the food on any overweight dog, trying some prescribed medication if your vet agrees, and if you can afford it, using some joint supplement such as Synoquin.