After his 2 month shots my sons legs swelled and he was very fussy\crying. What can I do to make his 4 month shots easier for him?

Anti-inflammatory. Would pretreat your child with tylenol (acetaminophen) or Motrin prior to next shots and for 24-48 hours after.
Tylenol (acetaminophen) You can give him tylenol (acetaminophen) before his appointment. Also, working his legs throughout the day and massaging the muscles where the shots are given (even though he may not like this very much) can help.
Tylenol (acetaminophen) Be sure to let your doctor know of any side effects from his shots. You can give child tylenol (acetaminophen) before going to get shots the next time to help with pain. Also, take along an ice pak to gently cool the area afterwards for just a few minutes.