Does he truly believe that there are monsters under the bed, or does he know that it's his imagination that is frightening him? I think these are subtly different things.
Certainly you need to acknowledge to him that his fear is real. But also that you and he can do something about it.
We never used monster spray, but I think it's a good idea.
Mine knew that the monsters were imaginary. I would ask "do you want to check in the xxx to see what's there?", and go along with whatever dc answered.
I would pull the dream out of their forehead/ears/tummy, roll it up, and discuss what we were going to do with it (with actions). Sometimes we would stamp on it and then flush it down the loo; sometimes we would throw it out of the window and a dustcart would drive over it and crush it to dust.
Two of mine had particular bears that had to sit in a particular corner to defend dc and prevent bad dreams. This was extra to the bear dcs cuddled.
I would tell a specific, made-up, very repetitive bedtime story that was intended to engage all their senses, eg involving lying in a hammock, feeling the mesh supporting you and the breeze brushing your skin, smelling the cut grass and hearing the creak of the hammock ropes as the hammock gently rocks, seeing the sun through the leaves of the trees supporting the hammock, tasting the cool lemonade etc. All the while repeating how all these things made them feel safe and secure. Sometimes the dc would fall asleep during the story, other times they would still be awake, but thinking about the story, and not thinking about monsters.
And any of our dc have always been welcome to creep in with us if they had a bad dream, at any age. (Still get the occasional 15yo!) If I'm still awake after a while, I do ask them if they're ready to return to their own bed, and I follow their lead on it.