Should I give my baby a regular bath or a sponge bath to lower his fever?

No. Fever is good, as it helps the body fight off infection. As long as baby looks happy and playful, you don't need to treat the fever at all. If s/he is fussy (and over 2 months), you can give Acetaminophen (tylenol). If it is bathtime and baby looks fine despite a fever, there is no reason s/he can't take a bath -- but no need to give the bath to specifically to lower the fever.
Yes. Either will help your child to be more comfortable. Use tepid not cold water since cold water will make your child shiver and raise the teperature even higher. Remember you aren't treating fever, you are treating discomfort.
No. Give your baby a bath as you usually would, not to lower the fever but rather for hygiene or if it makes him feel better. You do not need to bathe him to bring down the fever number. If your baby is not comfortable in the bath, skip the bath.
Yes. A regular bath is fine to do, to help a baby relax and get more comfortable. Parents should use tylenol (acetaminophen) or Motrin to lower a fever, instead of using bath water to cool a child down. Babies younger than 3 months, babies with high fevers, babies with pain, and those with more than mild ill symptoms should be evaluated by a doctor to see if there is a more serious problem going on.
No. When you last had a fever, do you remember how cold you felt? Can you imagine getting in a cold tub feeling like that? Your baby won't like it, either. If your baby is comfortable and feverish, you don't have to do anything. If they are miserable and feverish, try tylenol (acetaminophen) or motrin. If you baby is less than 2 months old with a fever, however, see a doctor immediately.
Yes. Usually a sponge bath is more comfortable than a regular bath, and it may help gently reduce your baby's temperature.