Thanks so much for the additional information.
It sounds like he may have arthritis in his hips. Actually your descripiton of his behavior is fairly classic for it: stiffness after laying down or sleeping for a while but normal gait after he works out the kinks, so to speak. Given his age, this wouldn't be surprising.
There are other conditions such as Diabetes or cardiac issues which might affect the back legs but your description doesn't fit with these problems.
Treatment options for arthritis in cats are more limited than for dogs but options to consider would be:
1 Cosequin for Cats which is a joint supplement. It comes in a powder which is sprinkled on wet food every day. It may be available at some pet/grain stores or on the internet.
2. Occasional use of anti-inflammatory drugs such as Onisor which is licensed for use in cats but can only be given for three days in a row.
There is a drug called Metacam but it is somewhat controversial in veterinay medicine, at least here in the States. It's use has been associated with significant damage to the kidneys and should be used with great caution in older cats; the drug currently carries a label to that effect. However, there are some recent studies which indicate that at very low doses, this drug may be beneficial for cat with osteoarthritis.
In addition, the drug sold in the UK is different than that which is available in the States and appears to be safer.
3. Adequan which is basically a stronger supplement but is an injection and needs to be given by your vet.
4. Fish oil supplements such as Welactin have been shown to have anti-inflammatory effects. It's a liquid that can be drizzled in the food. Also available on the internet.
5. Pain medication such as Buprenex can be very useful and could be given every day or only on the days that he needs it. This is a drug which would have to be dispensed by your vet, though.
6. NuCat Senior which is another oral supplement and a source of antioxidants to reduce oxidative damage to joints.
I hope this helps and that you see improvement soon if you elect some of the supplements that I've mentioned above. I should point out that these products can take several weeks to build up in the body so positive results might not be seen immediately but this may already be something that you know. Obviously, improvement is typically seen more quickly when drugs are given. Deb