How to deal with child's nightmares

[JJRJ1002]

Hi all

My ds is nearly 7 and for the last week has been petrified after seeing an advert with an alien on it and is too scared to go to bed and is been up lots in the night after having nightmares. The poor boy is hardly getting any sleep...and neither am I, especially when I have a 1 year old who is a bad sleeper.
Not only that but he is scared to go anywhere in the house on his own , he can't leave my side for a second. I have explained that what he saw wasn't real and that there is nothing to be scared of, but nothing is working. Does anyone please have any suggestions for the best way to help him and deal with his fear so that we can go back to normal of him being a happy boy and brilliant at going to sleep. Thank you x