Weaning - super constipated

LO has been on solids since 6 months old. He’s now 7.5 months. We do 2-3 meals a day and a mix of traditional weaning and BLW so sometimes he eats a bit more and sometimes a bit less. He has a variety of different veggies, fruits, proteins and grains and still takes his milk well - he has a combination of formula and breast milk. He also has a bit of water after every meal so has about 2-3oz a day. In the last week he has become super constipated. He strains so so hard and goes bright red and sometimes even cries in pain, only to poop out a tiny solid pebble each time. We’ve tried prune puree, oranges, more water etc. but nothing seems to be helping. What can I do to help him??