A stool of a dog on raw food is often crumbly ... if you leave it for a few days, it will often crumble into the soil. The crumblieness is not a bad sign, it's actually common and natural. The chances are that your prepared raw food comes with ground up bones.
I definately suggest you add a few raw meaty bones to his diet as well - the chewing/crunching is very good exercise and also a great teeth cleaner. Chicken wings are perfect in terms of the right bone/meat/fat ratio. If he's not used to eating meaty bones like that, he may try and gobble them whole! That may result in the chicken wing getting stuck in his throat. The best way to handle that is to hold the chicken wing in your hand and let him nibble it until he realises that he needs to chew/crunch as opposed to just swallow.
HOWEVER, if you feel he is constipated, then you need to add a few veges. The best way is to stick a combination of carrots, celery, greens etc and even a little apple/pear. (never any onion - check my blog for dos and don'ts) and liquidise with a little water. That will help him have slightly looser stools. Add some kelp for minerals - mix that all up with his raw food and he gets a very good meal. A few drops of raw apple cider vinegar is also very good as that helps with mineral absorption.
The suggested raw food mix is around 20-30% raw fruit and veg, but in liquidised format - because in the wild, dogs generally only eat this once it's been digested, and eat is as part of offal. So, if you can get hold of some offal and mix it in, that would be great too.