I cannot stress this enough, THINK SMALL. When my 6 foot long 3400 gram female got out, when I finally found her {and a snake that size you would think would be easy to see!} I saw a piece of her side that was no more than 3 ".
I'm not sure how big your guy is, but the thing to remember is they can get into the smallest of spaces. One thing to help you is the size of his hide. If for example he's small enough to use one of those cereal bowl hides, then he can fit anywhere you could fit one of those bowls. Stretched out, if he's the diameter of say a half-dollar, then he can fit anywhere you could put a coin that size, and if he's all stretched out, he'll be even harder to spot.
Start slowly, and close to where his enclosure is. Look up, down, inside, upside down, behind and between. Take your time, if you get the slightest bit impatient, you'll miss him. Look in the most unlikely of places, because that's usually where they are. I found that 6' long female wedges behind my rack, which is only about 2" from the wall. I still don't know how she squeezed back there.
I wish you all the best in your search.
Gale