I am not an expert certainly. If this were my tree I would make a batch of compost tea and spray it on the plant - trunk and leaves - at least once a week. Some nurseries/garden places sell compost tea.

It is tar spot, a common and pretty harmless maple affliction. It is typically only cosmetic and generally requires no treatment.

btw, the efficacy of compost tea for fungal or disease control is open to considerable debate. There are very few scientific studies that can provide evidence to support the contention that it has any affect at all. As a dilute liquid organic fertilizer, yes. As a disease suppressor, no.