My boy too have constipation.
I was also worried like you do.
well, i have tried giving him fruits , puree and lotsa of vegetable and fish .
now he is 10 monrh plus,
i gave him cod fish, salmon fish and veg of course for his porridge. daily differene fishes.
As for fruits, i gave him papaya and dragon fruits. but dragon fruits will cause a bit phlegm so not too much.
reduce his bisuits .
After his meal , can give him either fruits or puree .
until his stool have soften then reduce the fruits and puree amount.
now my boy only have 1 fruit per day and alternate day give his puree.
dont drink too much water as water will urine away instead of helping the stool to be soft --> this is what my pediatrician mention to me.
I do listen a bit , i still give him drink little bit of water .. haha