As this cut is since long between his toes, mostly might be due to fungal infection
as this is common site for growing fungus.
Now it seems that there is secondary bacterial infection took place giving rise to pus formation.
Go for one antibiotic medicine course to control infection after consulting your doctor.
Go for oral anti-fungal tablet like Flucanozole 150 mg once in a week for 4 weeks.
This will give complete relief from problem.
Apply anti-fungal, antibiotic cream locally after cleaning cut properly.
Keep local part clean,dry and airy.
Avoid wearing soaks for some time.
Ok and take care.