Sick Basset reason unknown

I have a male basset (Clifford Boomer) he is about 1 1/2 years old.....healthy as can be until this past Thursday evening.....he could not hardly walk at all, in fact i had to pick him up to get him to move.....he was not interested in much of anything including food....I took him to the vet the next day...she noted a temp of 104, swollen lymph nodes, she also took 2 x-rays that showed normal....she got his fever down by Saturday and had started him on antibiotics, which seemed to be helping until today.....he is back to just laying, it is very hard for him to even get up, he has ate today but very reluctantly, he is still relieving himself ok once he can finally get up and out the door... any ideas or suggestions would be greatly appreciated.....thanks

Lori