20 month old backing/delay

My son was diagnosed with an expressive speech delay at 15 months after we sought out services. He’s now 20 months and says all kinds of things, but only sounds at the back of his mouth and specifically avoids any mmm, bbb, or ddd sounds. I’ll include a list below to show examples. His SLP is stumped on how to help and has only told us this is abnormal development for his age. He doesn’t have a tongue tie and has had a swallow study with nothing wrong.

I guess my question is, should we seek a different SLP or have you seen this at all? I’m not even sure where he falls in a delay anymore since most of his language can’t really be understood by outsiders. His receptive language is outstanding and the SLP is not concerned about any other developmental issues. He points, signs, follows two step instructions, smiles and plays reciprocal “games” with us.

1. Gaga - mama
2. Gagy- mommy (two moms and distinguishes between each)
3. Gygy- bye bye
   - will say gyegye Gaga meaning bye bye mama if you know what you’re listening for
4. Gagy - Nabi for his grandmother
   5 - all - ball
   6 - coo coo for choo choo
   7 - cool
   8 - yay
   9 - ah uhn for all done
   10 - rou rou for round and round, wheels on the bus
   11 - go
   12 - gok gok - bok bok chicken
5. Gook for book
6. Gagy for baby