Birds don't have "scales" on their feet, it's just very thick skin allowing them to be able to walk/perch/hang etc. So it's not something that is going to "grow back" at all, it just has to callous/thicken over time. Since he had a wound and a scab there, that means that the full-thickness of the skin there was totally removed, and it's going to take a long time for it to thicken again. Nothing at all urgent or to worry about here, this happens anytime they get a wound on their feet or toes.
If you actually observe/see him picking at his foot constantly then that's a totally separate issue...Don't put a collar on him while you're waiting for that skin to thicken again, because that could take months and months, and there's no reason to have a collar on him all that time as it's normal for it to take that long to grow back...Now if you see him ACTIVELY chewing on his foot/feet then that's a very common nervous/neurotic behavior for Quakers to exhibit, it's akin to Plucking, and they usually end up hurting their feet and making them bleed badly. You'll be able to catch him if he's doing it enough that it's a problem...Otherwise it's just going to take time for that skin to thicken again...