Baby just had 6 month boosters. Fussy & lethargic. What can I do to make him feel better besides pedia-tylenol?

Bath. A warm bath sometimes helps. Beyond that, even if you don't use tylenol, (acetaminophen) he'll likely feel better within 24-48 hours. If he's not himself longer than that, see your doctor.
Patience. If it is a mild reaction to the vaccines, this usually passes in 24 hrs or so. If is a coincidental infection, other symptoms may develop. Call your doctor if this happens.