Should i mix baby cereal and prunes to avoid constipation?

I gave my son alittle bit of gerber organic brown rice cereal like the baby bullet recipe book advised for his first food mixed with breast milk. It looked like his normal milk, very thin. But he stopped pooping so I stopped giving it to him 2 days later he pooped and I haven't tried again since but he still only poops once a day now which isn't normal he usually poops ALOT. I've been reading online it's ok to mix pureed food with cereal for the first time...I wondered if prunes would help with the constipation if I pureed them and added them somehow? Thanks!