I wouldn't go back now-his body is in the process now of changing. But, I would put him on a better food than Purina. Purina is the very low end of good food-you want a midrange to start with. Iams is not good either. I would feed him twice per day-usually 12 hours apart or close to, and change his food over 4 weeks. 1st week 25% new food,75%old-second week 50/50-third 75% new, 25% old-fourth new food. I use nutro chicken, oatmeal and rice for sensitive stomachs and all of mine seem to tolerate this well. Sorry to say this, but you may have to put up with a few more weeks of mud pie poop.
As for the actual feeding-if he's hungry, he will eat. IMO To put him on a schedule is important to me, if mine don't eat within 15-20 min, it gets picked up until the next meal. I will give a few carrots or treats inbetween, but they get the idea very quickly that if they don't eat when the food is down, they have to wait until the next feeding time. Again, this is all MO only.
After I read the other post, I will add, rice is a good idea as well as pumpkin for his stomach, just not too much too fast otherwise you will have the opposite problem of constipation.