None.
If he wants to spend a lot of time in his room, just ensure that he is doing it from the joy of playing, not the avoidance of things that are hard for him to cope with. Handling difficult things in life is a huge source of real self-esteem.
No, I do not believe that it is appropriate for a 16mo to be playing with toys made (and safe) for older children.
If there was no brother to play with, what would the 16mo be doing? When brother's in his room, there is no brother --so what's the fascination with the door? When my kid were too far apart in abilities and interests to really play together much, I knew that entertaining the younger one was my job, not her's.
Having the 7yo in his room crying about what a mess the 16mo is making of his life and playing is no improvement over the 16mo crying outside the door. If someone has to be miserable, why is it the 7yo? Who's on his side?