My 4 month old son did not poop for 6 days now... help!?

Last 2 weeks ago i started feeding him cereals, but he got constipated so i stopped. he was really agitated and was in pain so i gave him something for pooping and did tried to stimulate his anus to poop.. then he did poop. it was really hard, the next day, he pooped again and its already normal.. since then he hasn't pooped until now (its been 6 days now). I am breastfeeding him and giving him 2-3 bottles of formula milk a day... (180 mls). He is not agitated just like before and not even looked like in pain. His tummy is just a bit bigger but still normal when i look at it...He is quite active now coz he already knows how to roll over.. i also tried exercising his legs maybe it will help... i didnt give him anything yet for pooping since he doesnt look like he is iritated or in pain. but, yes, i am worried. what should i do? should i start giving him something for pooping like prune juice? or is it normal?