4 year old to repeat pre-kindergarden

My son has just turned 4 and has problem with his speech. He is been seen by a speech therapist for twice a week for almost 2 months now. His biggest problem is the correct enunciation of words and some problem with the structure of sentences.

Although we have seen a lot of progress, the teacher in his school is worried that he does not like to play with other kids. She also has trouble understanding him when he talks. She suggested to delay his entrance to kindergarten for a year.

I find that the reason that he prefers playing alone is because the other kids are much older than him and they speak clearly. He was evaluated for autism or other disorders and the result was "highly unlikely".

My question is when will we see real results with speech therapy? And is it right to delay school for a year?