My 5 year old burneed his 2nd and 3rd digit on his hand the blisters have calloused will they heal on their own? The blisters a large what should I do

What kind of burn? Different kinds of burns do different thinks to skin tissue. How long ago did he get burned and what is the extent of the burn are important factors. Burns that blister are possibly second degree burns that require attention. Never "pop" blisters and keep the area very clean because of the infection risk. Decreased mobility may also be a problem. Have it checked.
Burned fingers. Burns of hands/fingers in young children should be seen by your doc, for if they extend across joints, contractures may form as the burns heal & compromise joint function.