For the past few weeks now my cat has seemed extremely agitated.
He is meowing an awful lot, and his tail seems to be twitching at an unusual rate. Also, his lower back just above his hind legs twitches too. My mum thinks this could be a sign of constipation? Is this true? It is not consistent twitching or meowing, but from time to time he seem very restless. He normally goes outside to do his business, so we don't know if he is getting rid of his waste, but his appetite and usual mannerisms seem the same, although he has become a lot more attached these past few weeks (following us everywhere we go, always wanting to sit ON us when we are on the sofa). Could this be a sign that he wants help?
We have checked him for fleas and that isn't a problem. He is a 16 year old tabby cat, and he was neutered as a kitten and we have had no problems in regards that area, if this information helps.
We are going to take him to the vets, but I would just like to know if any of these symptoms could potentially point to anything serious? Or if like my mum said, it could just be constipation. He seems to be in pain occasionally and it is upsetting.

The best bet is the vet, but as you say, he is sixteen and a senior citizen with some of the elder problems we have. He could be a little senile (the twitching), animals get that way too. We can't diagnose or make a call on this one for you. Animals rarely become constipated but he could have a blockage at his age.