teething is the cause of all of that, no doubt!
all of my children (6 so far) have had terrible acrid smelling poos when teething, with bottoms raw and bleeding like they just sat on a hedgehog! so you and DS have my sympathies! [smiles]
there's hope yet, here's what I do:
do not clean with wipes, it stings and aggravates the problem.
use only water and if very pooey showering him down might be easier, but don't use any soap or baby product, the best thing is a bit aqueous cream or E45 in place of soap.
if you are not at home wipe what you can off with a wet or dry piece of cloth or tissue, then use a bit of E45 again to get the rest of it off.
"airdry" his bottom and let him be without a nappy for at least 10 mins, I know it is a challenge, but try.
Most importantly put loads of Vaseline on his bottom before putting the nappy on, that is the only thing that always worked for my kiddies skin, and believe me I've tried a lot of stuff over the years!
do this at every nappy change, even if it's just pee, until the skin is healed.
at the the next teething start with the same method at the first sign of redness and hope for the best
if you know what foods would make him constipated normally, you could give him a small amount to reduce the "runnyness" and poo to skin contact to aid recovery.
wholemeal bread might do the trick or any foods with a lot of fibre that would normally plug him in will just balance things out.
for the actual teething pain I found Dentinox gel the best, you might want to avoid liquid Calpol & Nurofen, I found they badly affected the nappy content too.
good luck