Lilimama,
To start with a positive note, it's wonderful that your DS can talk (though at present he doesn't want to). Even if he does turn out to have a social communication disorder, you can help him develop the skills he needs so that hopefully one day he feels relaxed using language
As others have said, you need to see a developmental pediatrician. But as also said, diagnosis could take a long time. Diagnosis is not an aim in itself - just sometimes a necessary tool to make sure he gets the right help. Don't wait for diagnosis before reviewing the support he's getting and trying to increase/enhance it.
I would suggest you move this conversation to the special needs board where you will find many more experienced and sympathetic parents. You can also "meet" some autistic people there(amber is one, she is very nice)who post on mumsnet regularly.
Meanwhile, you might want to review the communication techniques you are using at home. It could be that if you use the very best techniques you can, you might find he is able and willing to communicate more than at present.
You might also want to tell us whether your DS is going to nursery/pre-school and if he's getting support there.
Others have more expertise than me, but I wondered if you are aware of the Hanen Foundation, a Canadian charity? They publish very high quality books both for "late talkers" and for children on the autistic spectrum aimed at helping parents use the best techniques for their particular child. I found that the quality of my communication with my DS2 (now 3.2, then 2.8, late talker and late understander too) improved within a few weeks of using their techniques - he hadn't changed, but I had.
good luck.