Hi there all, Please can someone let me know if...

Hi there all, Please can someone let me know if this is a common problem. My son who is 5 months old did not have a poo for 4 days and when he finally did it was really firm (about the consistency of playdough). We started solids about 3 weeks ago and this was going really well, so about a 1 & 1/2 weeks into it we moved to 2 solid feeds a day (fruit for lunch and veges for dinner). But now i am a concerned that maybe he wasnt ready for solids or perhaps i progressed too fast. I have been told to give him prune juice by a self professed expert and continue feeding him as per normal. Has anyone else experienced these problems and does anyone have any other advise..... I am starting to become quite stressed about the matter.