I don't think you are doing anything wrong, for some kids, it just takes longer.
BUT, only having speech therapy once a week doesn't sound right to me. I had a speech delay-well it's called Apraxia but because of that I was in speech 2-3 days a week and when I was in grade school I had speech therapy in school as well.
I saw the previous comment about Apraxia-What that is, is a neurological disorder that can affect one's speech, motor and sensory skills. It can affect one thing or up to all three. It is easier to diagnose now, then it was 30 years ago. A speech therapist should be able to help and tell you if it is Apraxia or not but if not, certainly another specialist. There are significant differences between a speech delay and Apraxia of Speech.
One thing you can try, if you aren't doing it already, is to work with the speech therapist and find out what you can do to help your son at home, continue those lessons with him.
Are they teaching him sign language? That can also help. At least with sign he'll be able to communicate, and when he learns the words he will use sign less and less.