How can I explain autism to my 5 year old?aut

There is a very complicated situation and I keep thinking about how to solve it. I will tell you a bit. Last week my husband’s whole family got together, this is something that doesn’t even happen at Christmas or holidays because my husband’s two brothers never have time to get together as a family, but anyways. My husband has two brothers and one of them has a son who is autistic, the boy is 7 years old and has very severe autism, my son is 5 years old and in all those years this is the second time they have seen each other. As I said before, none of my brothers-in-law visit us because nobody has time, but at the get together last week I couldn’t attend so my husband brought our two children to the meeting, that day apparently my 5-year-old son made a comment in front of everyone and said, “my cousin doesn’t want to play with me, I don’t know what’s wrong with him and he just wants to clap around the house” my son referred to his 7-year-old cousin who has autism and his mother quickly replied to my son that her son was not a normal child and for that reason he did not want to play with him. Three days later, my brother in law who is the father of the child with autism, had a conversation with my husband and referred to it by saying, “I do not tolerate people who is making fun of my son for being autistic.” He said that because of the comment that my son made that day on the get together, and he also mentioned that the boy’s mother was very upset by that comment which I totally understand her. I want to mention that on the day of the get together my husband apologized for the comment my son made and also apologized for the second time to them when everyone left. How can I deal with this situation? I feel that my son did not say it in an offensive way but I also do not tolerate my son making fun of anyone, much less someone artistic. What can I do? We haven’t say anything to our son yet, how can I explain this to him?