My five month old son is constipated. (He just started eating solid foods.) Any tips or ideas on how to get his bowel movements back to normal?

A little juice. If your son is having stools that are soft then he is likely not constipated, even if he is only having a bowel movement every couple of days. The introduction of solid foods can change the frequency of bowel movements as the solid foods take longer to process than milk or formula. If his stools are very firm, adding a small amount of juice to his bottles may help.
Hard stools. Constipation in infants and toddlers is a common problem. It is usually temporary, but if left unaddressed can lead to undesirable toileting behaviors, discomfort and distress. Increasing water intake, offering high fiber juices (peach, pear, prune) and limiting yogurt and cheese may help. Occassionally, pediatric Fleets enemas or glycerin suppositories are required.
Fresh fruits. Give fresh fruits mashed up BUT avoid bananas. Sometimes fresh pear, prune, peas, peaches pureed enough as to avoid choking really gets things "moving". Also, fruit juices--undiluted generally do the trick. If things worsen see you pediatrician.