My husband found a soft fluid filled bump on his side. when you push the center of it, it's sore. what could it be?

Fluid filled bump: This is a tough question to answer without more details. A soft fluid filled bump may be a blister from trauma or burn, a seroma, an abscess or infections. If it is red surrounding the area, it is likely an infection, particularly if he is having fevers. A bite is also possible. He may not be aware of any antecedent events. Probably saftest to have it examined by a physician.