I have this exact scenario with my EBF 7&1/2 month old. He took to food like a duck to water, but any amount constipates him.
Tried bicycle legs every hour to keep mobility up, tummy massage a few times a day, water in a sippy cup with every meal, having each meal have a high fiber component. His diet was basically prunes, pears, peas and broccoli for two weeks and it didn't help. I tried keeping it drippy enough to run off the spoon, and still didn't work.
He's been to the GP and is now on lactulose every other day. The GP said that some babies just take a while to adapt to solids. That it was more important to keep things moving, and there would be no lasting harm from using lactulose regularly. She said to revisit it after a month, and if he needs it daily to provide it. She was worried he was starting to withhold the poos out of fear of them, and I think she was right. She also said to try 2-3 oz of juice middle of the day, and to make sure there was lots of fluid in his tummy with the lactulose as it would pull the fluids in to his bowel.
I've also found that having fats with every meal helps a little. It softens the stool almost, though too much can slow digestion.
Plus avoiding baby cereals, as the added iron can be constipating. Also avoiding binding foods like banana, apple sauce or eggs.
When he does go to poo, I sometimes help massage it out by pressing the the perinium (sorry TMI) because I find he'll get half out and then stop because he doesn't like it.
He's now on three meals a day, and having a higher volume of food hasn't changed anything. I was trying to go slower, but he's a hungry baby and literally screams like a baby bird for it.
Hope it resolved for you soon!