Metronidazole is often used in cases of GI problems because it has two benefits, not only is it an antibiotic but naturally it improves stool quality. In many cases of chronic GI upset, animals are placed on this medication long term to help with stool issues. The fact that it helped with the blood, but not the diarrhea, may suggest that another antibiotic will need to be used. My personal favorite for chronic diarrhea in adult cats is baytril. This is not usually one that's reached for, but I've found it to be very useful with those diarrhea cases that don't respond to much else. In those cats I've had come through my own home and rescue that had cultures performed on their stool after chronic diarrhea battles, baytril has always been the magic medication. Two cents from the trenches. Might save you some money in the long run.
If your boy is not already on a GI pre/probiotic, now would be the time. It won't hurt to add this into a regimen to see if it helps. The point of this supplement is to get the proper balances back to the GI flora. It's often that something can throw off the balance and that's when diarrhea starts. It's never been a fix-all in my experience, but it has helped greatly those who needed it: http://www.1800petmeds.com/Pre+++Probiotic+Water+Soluble+Powder-prod11655.html
Another item that may help is to use some additional fiber. This is commonly recommended in veterinary medicine, although I have yet to see it make astounding improvements for cats. That being said, it's clear that since he's passed a hairball that it may assist him more than your typical feline. You can use this now and in the future. You cannot overdo it with fiber and cats who do have long coats typically benefit greatly from the increased fiber in their diet. I use and recommend Libby's canned pumpkin. You can buy organic sourced canned pumpkin, by preference, but ensure that whatever you use that it's just pumpkin and not pie filler. Cats typically enjoy canned pumpkin and many of them will eat it plain. You can also mix it in with the canned food, if you'd like. You may notice his stool take on an orange appearance and it may stain his haircoat (probably not worse than the diarrhea he's already experiencing, thankfully). Because most cats only need a little and there's a lot in a can, don't hesitate to open a can, put a tbsp or two into each cavity of an ice tray and freeze them for future usage. A few tbsp is plenty per cat each day, so you can fill the ice tray cavities either halfway or all the way to the top. Either of these amounts will be fine for your boy. Again, can't overdo it with the pumpkin. The worst you'll have is orange stool.
Eventually, your vet is going to recommend doing an out-house lab, parasite screen and culture and sensitivity. If your boy is not exposed to other animals, the risk of parasites will be low, but a culture and sensitivity can yield useful information in telling you what bacteria are present, appear to be overgrown and what antibiotics you can use (see above with my baytril reference). This step is most useful in determining if a diet change should be done (see below).
Although it would be uncommon, you may also have to look at a diet change. It's not uncommon for purebred cats to acquire food sensitivities and IBD with age. Although he's only two, he's old enough to have inflammatory changes in his bowels. I won't harp on this since we don't know if this will be the case or not, but keep the potential of 1) limited ingredient food in your back pocket and 2) a raw diet of novel variety (like rabbit, duck or turkey). Rather than go into vast detail, it might be easier to reach out to me again later if you make it to a point where you're ready to throw up your hands and try a change of diet. I'll be happy to write you a novel then on all the things that I've seen purebreds tend to respond to when the typical diets fail. Most vets will recommend Hill's I/D, W/D and so forth. I have a sneaking suspicion since you feed Wellness and Natural Balance that the high filler content in commercial veterinary prescription foods may not sit well with your desired food quality. If that's the case, we can work around that. They aren't the only company to make sensitive stomach diets.
There's a lot more that I can add to this, but it will help me to know if your boy makes improvements with the first steps of pre/probiotics, added fiber and if a switch to baytril did him any good (and you can certainly try others, baytril is just the one that I prefer to reach for since it's been a drug of choice on practically all of our culture and sensitivity reports). If not, I'll be happy to talk you through our diet routine for purebreds with chronic stool issues. So far, we've had over 50 purebreds surrendered for this condition and every single one of them has made the switch to normal stool and they've gone on to be adopted. Best of luck with your little man <3