Which is the most effective treatment for molluscus contagiousum in a 4 year old boy (my son has had a few bumps for months behind his knees)?

No treatment. Molluscum contagiousum is a viral illness, and presuming that he has a normal immune system, should not require any treatment. The lesions will generally go away in about 6 months. They can last longer than that, sometimes for years, though most will clear in a period of months.
VIRAL INFECTION. If you can not wait for months for it to disappear, just pop the lesions and something white will come out like pimples. The lesions will dry up and disappear.