Baby constipation

My little one is going on 13 days without a poop. It is normal for him to go a week+ but this is the longest he has gone. Our pediatrician recommended giving .5oz prune juice... wait a half hour and give .5oz again if it doesnt work. Well I gave him .5oz last night and .5oz this morning followed by answer .5oz a half hour after but nothing! Does anybody have other recommendations? She told me I could try gripe water or probiotics if I wanted but prune juice would have the most success as those remedies are not for constipation. He is not in extreem discomfort but definitely more irritable the past couple day.