Does Oliver scootch his bum across the carpet, as if it was itchy? That's ususally a good indication that he doesn't have enough fibre since it likely means that the anal glands aren't being evacuated when he has a BM. When the glands are full, they're uncomfortable for the dog.
I agree with DarKevs, pumpkin is great and dogs seem to like it. Just make sure you get pure pumpkin and not pumpkin pie filling (that's got sugar and other suff you don't want to be giving to your dog). Other forms of fibre are apples and carrots.
Have you also considered whether Oliver has enough beneficial bacteria in his GI tract to adequately digest and absorb his food? If he was a stray before he got rescued, there's no telling what's in his GI tract. Go with plain, Balkan or Greek style yogurt, no flavours, no sugars, sweeteners or colours.
I give my 23 lbs Cocker Spaniel 1 heaping tablespoon of yogurt in the morning and 1 heaping tablespoon of pumpkin in the evening with her food. I reduce the amount of kibble she gets to the low side of the daily amount for her weight to compensate.
If you're green, you're growing...if you're ripe, you're rotting!
I try to greet the world like my pets do...boundless enthusiasm, intense curiosity and no concept of yesterday and tomorrow.