In that case, I'd suggest placing the seal on his dreams rather than yours, since he is the one who is entering uninvited. You can do this by setting a "trap" within yours. While awake, intend a place within your dreams that he'd be drawn to when entering. Cast a circle there that will become active when you say a key word. When he enters that space in your dream yell the word and cast the circle around him to bind him and cast him out. Have the circle go with him as a seal to prevent him from further intrusions.
Keep in mind that the energy that you send out also returns and should therefore be done without malice.
Good Luck and Blessings,
Dilas
