Just think, if you help your son understand himself now, all the years of self loathing and struggle you’ll be saving him.
This, exactly, OP - you've done him a huge favour getting him assessed in his teens as opposed to letting him struggle undiagnosed into adulthood feeling there must be something wrong with him.
My 3 were each diagnosed as they hit puberty and I was diagnosed in my 40s this year. My NHS support group (I know!!) for newly diagnosed adults is full of people coming to terms with their diagnosis and one of them said the other day "I dislike myself a little less each day now" which sums up how I feel about my diagnosis too. So you've saved your DS a great deal of confusion and heartache and unnecessary self-esteem issues by giving him an explanation for some of his struggles.
My eldest DD was terrified of people finding out she was autistic for literally years after her diagnosis, but she is now in her 20s and openly talks about it and is far more comfortable with it. It took years, though, and I think it helped for her to see how addressing her needs actually benefitted her and made her life much easier before she got comfortable with it.
I think with telling DS about the diagnosis, a helpful way to frame it might be to explain that lots of people have autistic traits, and that the criteria for a diagnosis is based on how much those traits cause difficulties with daily life compared to someone without those traits, since the world is designed by people without those traits who had no idea that other people might struggle.
So while the diagnosis might seem irrelevant or unnecessary or even unfair to DS now, there might be times in the future where he might benefit from being able to understand how he thinks slightly differently from how others do, or where autism can often be a useful shorthand for explaining a whole range of traits to someone to help make his life in school, work or leisure activities a bit less stressful.
And he doesn't have to "come out" to people in any big way. I haven't even told many people about my diagnosis unless it's relevant or I want to - he can just take his time to learn what it means for him and acknowledge it or ignore it as he feels comfortable.