Hi Traci
I wouldn't get too worried about the liquid-poo yet. I would take away the yogurt, too, personally, since adding dairy may not help if he's upset because of the new food issues. If he won't eat his familiar dry food plain, then try to feed the same flavor pouch with it every time for the next week at the very least. Worry about his "forever" diet later, you just want to get him back on track.
I do have to put a vote in the "high quality" food column for the long term. I think it's reasonable to think that food quality matters to overall long term health. (Sick cats get to what what they'll eat, though, that's the rule.) I have $$ issues too so I usually go to petfooddirect.com with a 20-22% off coupon and search for high quality foods (no by products, no mystery ingredients, identified meats as the first 2-3 ingredients) on sale. I can usually find stuff for under $20 for a case of 24 6oz cans, or 12 13 oz cans (these tend to be even cheaper). I have noticed that the cats are more sated on the higher-quality foods, actually. The coupon usually covers shipping costs.
But that's neither here nor there for your current dilemma. If the explosions continue you should try to get him in so that you can start metronidazole; you should also, when the poo gets a little better, submit a sample for a fecal test at the vet just to be sure. If you can't get in tomorrow I wouldn't panic as long as he is eating, not vomiting, and not apparently dehydrated ok? Hang in there, he's probably just got a bit of colitis from stress and the food change.
PS You may want to see if they can trim his fur in the rear, if he is still having major issues when you take him in. Longer-haired cats can get a little messy.