It doesn't sound like he's especially healthy, but nearly every malnourished rescue kitten I've bottle fed has had awful stool and an inflamed butt for a few days after going from NO food to rich, unusual formula. I would not panic, but I would give him a little squirt of nutri cal a few times a day, and try to get some extra liquidy formula in him if you can. Most kittens seem to do ok with a little dab of nutri cal.
BTW no kitten eating formula is going to have firm stool. At best it'll be a soft "tube" of poop. TMI I know but I've never ever had a kitten with hard poops on formula. I do get better results with homemade formula than I do with the dry powder stuff though.
He does need to be wormed and checked out, though, I agree. See if any rescues can help you out.