my daughter was seen by a Speech and Lang therapist at 15 months and to start with they wondered if she was Autistic. As time went on though she became more "interactive" and that was crossed off the list and now (she is 5.5) we still have no concrete diagnosis.
She has severe speech and language problems - so she is behind her peers in being able to understand, her speech is difficult to understand and some things wwhich seem like common sense, she needs to be explcitly taught. She also has dyspraxia so finds motor skills difficult (eg peddling a bike, holding a pencil, cutting with scissors etc). She is a model student - doing as she is told, cheerful and outgoing in class and trying her best at all she does. She is also a popular little girl with the other children all having a soft spot for her.
We don't know what the future holds for her. She comes on in leaps and bounds and is the happiest most, determined independant little girl and such fun to be around. She can read a little (so is about average for her age), has managed over the last 6 months to learn to write legibly and is beginning to try and form grammatically correct sentences.
It is terrible when you hear there is a problem with your child - and even worse when you get no clear dignosis and spend hours poring over the internet trying to self diagnose. I think it is the worse time in my life when we went through that as you have no idea where to turn or what is going to happen.
It does get easier - and having severe SLI does not necessarily mean your son is autistic - as my daughtr isnt. And unfortunately it doesnt mean that anyone can tell you where you will be in 5 years. They may still be very behind, they may have caught up. It may barely affect their life, it may make things much harder. There is no clear answer I'm afraid.
But it DOES get better as you become more aware of the help you and your son can receive - and the fact there are others who have been there and can advise you. Good luck.