My DS now 1 year old has suffered with constipation since he was born. Sounds like you’ve had some good advice. Things I follow:
Lots of ‘P’ foods - prune, peaches, pears, peas, pumpkin etc.
Fibre - Oats and weetabix are good when you’re just starting weaning. Chia and Lentils are also great! My DS loves lentils and find they always make him do a big 💩 but ensure to drink water with foods high in fibre or combine with juicy fruit (peaches, watermelon, mandarins, oranges, berries).
Increase water intake - my HV said as he’s formula fed he can be offered water between meals rather than just at meal times.
Avoid binding foods - bananas, yoghurt, eggs and cheese. A little is fine for calcium etc
supportive measures - bicycle legs, warm baths, tummy massage etc.
Hope your DS doesn’t suffer too much. We have ended up on laxatives only because the above measures are not always enough for him but without following he is much worse.