By 8 months, my kids were eating all kinds of wonderful regular food. I was lucky, they didn't have any allergies and their tummies handled everything. I wouldn't have known what to do because they loved food!
The real thing!
They HATED the blandness of baby food.
Prunes really aren't that bad, I don't think. It's a personal preference I guess. They are just dried plums which is one of my favorite fruits of all time.
If your son loves prunes, let him have them. Some kids go on different kicks as far as fruits, melons, etc. As long as he likes it and your pediatrician says it's fine, let him have the prunes regardless of how you think they taste.
Another thing is that my daughter was born full term, but she walked before she could sit up by herself. She had what we called a "round bottom". She had no interest in sitting alone, but she did fine in her high chair. Sometimes I sat her on my lap and let her takes bites of our food.
If you're worried about spices, maybe take a portion of things out before you spice them. For instance, if you make eggs, take some out before you salt and pepper. Or add garlic, etc.
I'm not sure your son necessarily isn't digesting his food. It's just having trouble coming out the other end.
I would try giving him some regular food, keep up with the prunes, and see what happens. Add a thing or two at a time so if he has a reaction of some type, you can narrow it down easily.
I nursed my son for 15 months and he definitely ate 3 meals a day as well.
I'm not an expert, just know what worked with my little ones and it's possible getting a variety of other things that aren't binding might help his tummy some.
Best wishes.