He found his glasses crawling under the couch.
Were his glasses really crawling under the couch?? I'm sure not!
How can you rephrase this sentence so it's obvious that HE was the one crawling under the couch?
??
To rephrase the sentence and make it clear that it was HE who was crawling under the couch, you can say:
"He was crawling under the couch when he found his glasses."