We went through something similar - we knew ds2 had S&L difficulties, first raised at 18months - he was delayed in other areas as well - so we had the benefit of a play therapist for a while and another lady who came in to help with communication.
Speech continued to be significantly delayed and eventually we were lucky to gain a place at ICAN Nursery when DS2 was 3. He attended this and staff raised concerns about routines, etc, etc so we agreed to proceed with an assessment.
We watched his every move - it was horrendous; is he?; isn't he? - oh he wouldn't do that if he was asd, or yes, that's definitely asd - it was totally stressful and I cried buckets.
Then we finally had an assessment in Dec 05 and, although, there were definitely S&L difficultes he wasn't diagnosed with an asd.
So that was fine for a few months, his school (now in Communication Unit attached to mainstream school) continued to raise concerns and eventually after months of the is, he isn't he again, we got a dx in Feb 07 of HFA.
The waiting was dreadful, but the dx was easier to make because he was a bit older and his difficulties were obviously not due to S&L by then.
Check out what resources are available to you - nursery, perhaps specialist if necessary, schools, etc.
You may be lucky to get something definitive from the psycholgist who specialises in asd or, if you are like us, it might be a case of watch and wait - hard as it was we still had all the services we needed on board which is a big comfort tbh.
DS2 is now 8, his speech has improved so much - although we are still working on it - he is reluctant! and there are only 7 other pupils in his class and he is very happy at school.