I had a rottie that used to get nervous and would lick his paw, till it was sore than it would get infected. Is he licking his paw a lot? If so it will never heal. Try putting a old sock on it or if necessary on of those collars that stops them from being able to. I spent a lot on that dog for licking his paw. Every time antibiotics where about $150 to $200
My rottie had a nail bed infected a few months ago. Im pretty sure it was from digging up woodchuck holes.
The vet gave me an antibiotic and told me to soak his food in epsom salt 2 times a day for 10 to 15 mins.
That seemed to take care of it pretty quickly.
OharaTree, may be correct about something being in there. Or it could be a cracked nail you can't see. I treated one dog with a water bleach mix about 25% bleach and 75% water, I dipped the whole toe and pad area for about three or four minutes, washed this off and put peroxide on, then some athletes foot medicine, it healed fine but I don't know which one helped it. i thought it was a fungal infection. Maybe it just healed itself. The one with the cracked nail I cut the nail way back and coated the nail with crazy glue.