Scrotal tissue being a sensitive dependent part may develop some itching as it touches ground quite often. Your dog's pimple type scrotal growth may be due to contact dermatitis, physical trauma, bacterial or fungal infection.
Next Steps
Licking scrotal tissue aggregates it and makes it perpetual. Try to avoid licking , so as to keep it dry, with the help of E-collar if it's so.
Health Tips
See your vet once again. It's a simple problem to resolve in the beginning. Delay in treatment and constant licking makes it worse. Hope this helps. thanks