Looking for advice - 10 year old rude to younger children

Hello, I'm new.

I'm looking for some advice and to see if anyone has had any similar issues with their child or has been in the same position as my eldest son and can help me to understand whatever I'm missing.

I have a 10 year old son with high functioning moderate Autism. He has a younger brother who also has a similar diagnosis. 

My eldest son has always had lots of friends of his own age and has always been well loved at school by children, teachers and parents despite him taking some things literally and getting confused sometimes. He has always been a kind and loving boy.

His younger brother took a long time to make friends at school. When he eventually did and his friend started to come to play I realised to my horror that my elder son was not very tolerant of his younger brother's friend which seemed completely put of character for him. At this point my younger son and his friend were about 6. My elder son would interfere with their games and would try to influence his brother to do what he wanted instead of playing with his friend. He would be difficult about letting them play on things and sometimes just says hurtful things to the younger child. It seems to be just the one friend that he does this to. I know this particular friend has rubbed other children up the wrong way but I have had so many chats with my eldest son about this, explaining that he's the older child and that he needs to be kind and that theres no need to be mean. I've suggested that my older son goes to his grandparents for the day when my younger son has his friend around but my elder son insists on being at home because he feels he's being left out otherwise. The latest play date went ok for the most part until my elder son openly told the younger child that no one liked him. When I told him off and spoke to him later he said he was just being sarcastic but he clearly doesn't know what that means. 

I feel awful for my younger son (and his friend too) because he struggled enough to make friends without his brother interfering. 

I'm worried for my elder son because this is unacceptable and he starts secondary school this year where people don't tolerate much. He's at a specialist school but I still worry that he'll struggle. It also not something we've had to deal with before and I really don't know what to do. My instinct is to be strict but so far that only seems to be making matters worse. When I talk to my older son he just says "this is how I am". 

Any advice would be gratefully received!