I've been having the same problem with DS4. And I'm realizing after having him evaluated by 3 speech pathologists on 3 different occasions that regular speech pathologists don't seem to pick up on these things unless maybe they'd be with the child every single day?
I had been so frustrated trying to get him to understand certain things over and over and over and nothing seemed to work. And then couple weeks ago he started watching Daniel Tiger neighborhood show on the PBS kids tv and it's been doing wonders for us! Maybe just watch one of the shows yourself and see if it's something your son would be interested in watching? It's not just the things they teach on the show but the way they teach them .... DS4 really seems to get EVERYTHING they say!
... I had to laugh so hard yesterday ... DS4 was playing with play-doh in the living room and suddenly I hear him say ... "hm, the wall is green!" ... I jumped up since the wall is light cream color! ... go into the living room and see neon green play-doh spot on the wall! So, my immediate reaction is "what you did is very BAD! Do not do this again! Do you understand?" ... and my dear child responds with a word by word with a song quote from the Daniel Tiger show: "When something seems bad, turn it around, and find something good!" ... He was so serious telling me, like he fully understood what it meant! ... and it's times like these where I'm realizing he's finally starting to "get" some of the things we say that I never thought he would get

anyways, don't have much advice for you ... but I know how you feel!