legs give up back pain numbness of feet

my uncle for few months now have been haveing problems with his back legs feet,when he stands up for a period of time his legs give up to the point he needs fall over,he has no refel in his feet they are numbe he does not feel anything in them,he as sever pain in  the bottom of his back,however when he is out shopping in less he holds on to a trolley and bends over he seems fine,it sounds like a bit of condition what my father motor neurone disease but drs has tested him for thet and ruled it out,i think it is something related to motor neuron disease but cant seem to find the answer i am lookign for the conditons he is suffering,is also gives me the signs of mad cow disease how his legs go but he walks fine with mad cow disease your legs tend to wobble well his doesnt they just give in from under any idease thankyou julie