He is significantly behind other children, this school is not a best place for him and they don't have resources to look after him. They are not interested in listening to us.
As a former teacher of children your son's age I have to say this is a very unusual thing for the school to say to you and suggests to me that either you have a terrible teacher or there is something wrong with your son that you are not seeing because you perhaps haven't interacted for long periods with other children of his age. I'd be inclined to the latter only because I have never met a teacher who would want to say the above about any child that wasn't causing a huge issue within the classroom. Most teachers are kind and compassionate and really care very deeply about the children in their care. It always broke my heart when I saw a child who was clearly experiencing difficulties which the parents refused to acknowledge. It's also impossible for schools to get rid of pupils with all but the most extreme difficulties so either he is extreme or they are exaggerating for some reason.
Either way please please allow them to get in the educational psychologist for an assessment. This will have one of several effects. They will find he is autistic (very unlikely at the first observation anyway as these diagnosis are rarely handed out and take a lot of time) and he will get the help he needs. They will find he is not autistic and the teacher will be reassured that with more support from a TA he will catch up with other children, thus putting her mind at rest and you will feel vindicated. Alternatively they will find he is probably not autistic but that he does seem to have different additional learning needs that need more than in class support - he will then be assessed further and will get the help he needs at this early stage rather than waiting until damage has been done to his learning. Surely you would want him to get the best possible start to his school career and therefore you should definitely go ahead with the assessment, which can only result in a positive outcome for your child.
If you honestly can't go through with this then another school is your only alternative. If they too come to you with concerns you will know that they are right. If they don't - well it may be that they are just bad a picking these things up and you may find out a later stage that something should have been done sooner. Of course they might pick nothing up because there's nothing wrong. It's a risk you would be taking.
I'm sorry if this sounds harsh because I'm sure it's horrible to be told there is something wrong with your child, and teachers - particularly inexperienced ones - can make mistakes, however I really do think that the best option for you and your child is to get an expert opinion in just in case. An educational psychologist will be completely independent of the school and won't be influenced at all by what the school say so you can be reassured by what they report either way.
On a another final note you are within your rights to ask for his current assessment scores from the school and for written details of exactly what he is behind in and why they think he is autistic (what signs/examples they have noticed). If you can get this then you may be able to check his results against the national average yourself, and compare what he can do at home with what he can do in school more accurately, or post here for advice from others.
Sorry for the long, rambley post. It may turn out to be the school with the problem rather than your son, but it's definitely worth checking just in case.