Aging badly or something else?

Question
My 2 yr old rat, Sweet Pea, has always been a tad shyer than other rats, but recently ( last 3 days) he has been acting very cautious. He seems to have "slowed down" in almost everything he does, including/especially eating though he drinks huge amounts of water. He eats mostly when I feed it to him bite by bite. He can walk but his hind legs are showing signs of weakness (occasionally he has to drag them before he gets the hand of walking again. He has become overly submissive to my other rat as well. He has not been stretching out his neck. His extremities are not turning blue and he does not have a respitory infection (vet checked). He seems almost okay when I take him out, but within ten minutes he's crawled back to his bed. His urine has occasionally been brownish red or orange (happens periodically but never diagnosed as anything by the vet), but for the most part both urine and feces appear normal.

With all these things together, could it just be rough aging or is it something else?

Answer
Could be cardiac related. The vet needs to do a better exam.  The heart can also cause them to have problems with their hind legs. It could also be neurological.  I am not real wild about how he drinks alot, either.  How educated is your vet on rat health?