Theres probably a hundred ways to do this, it can get a little complicated when you need it to automatically close as well, and then reopen if he walks back into it but you can do something like this
In the door blueprint (if its not in a blueprint put it in a blueprint) make an event or function with the functionality of opening the door
Add a collision sphere component to the blueprint
Add an OnCollisionBeginOverlap Function for the collision sphere
drag out the bluepin that says other > Cast to Character Blueprint > then call the function for opening your door