I honestly think once you get his gut healed (it may take several months) he will be able to eat the jerky and a bigger variety of kibble. I suspect this was going on before he was rescued and right now he just needs food that is very easily digestible so as not to irritate his gut. If you do try to introduce any of the duck and potato kibble or treats, I would do it very slowly with only 3 or 4 kibbles or half a treat in a day. I think he will be able to digest it, but take it slow to give his gut time to adjust.
I tried to find out how much soluble fiber was in several different brands of kibble (including NB), but all the companies could only give me their total fiber. Since you said probiotics and kefir don’t seem to be making much difference, that makes me think that his diet may not have enough soluble fiber to feed the bacteria (based on my experience with what I saw in my girls). I personally prefer adding food to the diet although you could add a probiotic with a prebiotic in it.
If you continue to not see any improvement (firmer, smaller, and less frequent poos), I think it is worth a try add a little banana along with the kefir and see if that helps any. Like you said, give him some time to stabilize before making any additions.
