Hi thanks for your question.
Your have fractured you Humerus
at two places and orthopedician fixed you fracture with DCP (Decompression plate) and put a splint
on the fractured arm.
The treatment you received is absolutely right.
The aim of big splint is to immobilize the joints proximal and distal to fracture, so that there is no movement at all at point of fracture
As for as care of the incision wound is concerned, if he has made a window in splint, you have to dress it on alternate day. If he has not made window, there is nothing to worry, incision wound will heal of itself, inside the splint.
Removal of splint depends on condition of fracture and is usually removed after 4 to 5 weeks for upper arm,
You can consult orthopedic an back at your home and take his advice in this regard.
For pain you can take painkiller like buprofen.
Hope this answers your question.