Is it okay to take expired cephalexin(500mg)?

What if it is a liquid antibiotic prescribed to my son it is only 2 months old but I fever he is getting this same respiratory infection and this is what an ER doctor gave him?

Yes, it certainly is. There is a great deal of research on this subject. Many studies have been conducted over the years which prove most prescription drugs remain quite potent and in fact, do not degrade years, sometimes even decades, after the labeled expiration date.

NO, it is NOT okay to take an expired prescription.
You might try a triple antibiotic ointment other the counter and see if that helps, otherwise you'll need to make a doctor's appointment and get an rx for it.
Drugs have expiration dates because they degrade.
Don't fool with any expired drugs or unidentified prescription pills you have laying around.