Rashy reaction to sweet potatoes - help please

My DS has a rash all over his body that I'm pretty sure is a reaction to sweet potatoes. I have taken him off of them, but the rash is slow to go away. I think benydryl would help him, and the doctor said we could have some on hand, but I'm not sure if he meant the cream, or the actual suspension liquid. I would like to give him the liquid, but I'm not sure how much to give him. Have any of you girls given your LO's benedryl liquid? DH is 4 1/2 months now....just not sure what to do. Poor little guy is all kinds of itchy and fussy. Makes it hard for him to sleep.