possible constipation- 6 months 10 days

I introduced solids 3 weeks ago. starting with oatmeal cereal, then butternut squash, carrots, sweet potatoes, avocado and peas. at avocado and peas ( 4 days ago..ending avo on peas) his daily poo count dropped from 3 times a day to one. Today he has not had one stool and it's almost 6pm. Went and got prune Gerber food today and gave it to him at 4:30pm. He is starting to cry and whine. I have massaged his tummy, done legs kicks. ...any other suggestions everyone? I breast feed and he feeds at 7am, 11am, 2:30pm and 6 pm so four times a day with the solid food given at 12 post 11am feed.