You could try a homeopathic remedy found effective to open the abscess to drain it. 8x (Gunpowder) homeopathic remedy has antibiotic properties. There is a stronger formulation (30C) which as noted is at the lower end of the higher doses used by wildlife rehabbers for stubborn infections. Having this as part of the arsenal to counter ill health conditions in squirrels can lend significant support towards healing and restoration.
These links are for reference on homeopathic remedy use in wildlife.
https://www.ewildagain.org/homeopath...-tips-wildlife
https://www.ewildagain.org/wildlife-...y-is-different