Explaining Autism to my son

My son has just finally been diagnosed after 3 years of hitting our heads againstr brick walls. he has always had an understanding that he is different in some ways. I have now been able to say it is autism. What I am finding now though is whenever he is naughty or doesn't do his handwriting neat at school he will say "But I have autism". I have tried to explain that this isn't an excuse for bad behaviour or sloppy work but it is a reason why some things are harder for him. How can I stop him using his autism like this. I want him to understand some of his difficulties but not to think "oh well I don't have to bother cos I've got autism" He is high functioning and is not at all anxious socially, in fact he is the opposite and just doesn't get the social cues to back off and give people space. Any help or advice would be welcome, I want to be sure that I do the best I can for him.