Is this the show's intention, to make the viewer think that Bill's son is autistic?
I don't think Brian is necessarily intended to be portrayed as autistic, but instead, just has considerable social & communication issues.
For example, in episode 6, when Holden visits Tinch's house for dinner, he meets Brian. Upon meeting Brian, Holden sits very close to him, touches & plays with Brian's toys, and knocks over Brian's tower. But despite Holden being so intrusive, Brian doesn't appear to get anxious at all, nor does he get upset at any point. Also, when Holden eventually walks away, he put his hand on Brian's head and rubs it, but Brian doesn't even flinch or change behavior.
Generally speaking, some amount of anxiety is demonstrated in autistics, but Brian doesn't seem to show any. Given how aggressive Holden was when meeting Brian, although Brian didn't even acknowledge Holden (or his father for that matter), I feel like, if Brian was meant to be portrayed as autistic, he would have exhibited at least some change in behavior to emphasize this fact.
Really, there's nothing that specifically suggests Brian is autistic. He could have a social disorder, be traumatized from a previous event (e.g., loss of sibling), or just have issues with his father never being around. When considering how Brian behaves with his mother, he seems quite responsive and compliant.
The only other time Brian is shown is in episode 4, when Holden is picking up Tench from his house. Brian is called by his mother to hug Bill, but Brian just stands there. Again, nothing here directly suggests autism.
As a side note, the "purpose" of Brian's social deficiencies could be to serve a platform for Bill Tench to reflect on his own actions as a father, and the potential effects that parenting may have on their children committing "disturbing behaviors" (a term previously coined by Holden).
Example: After dinner (still episode 6), Holden and Tench are discussing potential patterns in their subjects, intertwined with some of Tench's personal life:
TENCH: They all have a crazy, angry mother.
HOLDEN: Or an absent father.
TENCH: Aren't all fathers absent in some way? I know my old man was never around.
HOLDEN: Imagine how different Benjamin and Rose's lives might have been if their father never left.
TENCH: Not sure it would've changed much.
HOLDEN: Your father really didn't speak to you?
HOLDEN: But you speak with Brian?
TENCH: You want one more?
From this conversation, it's clear that Tench is reflecting on how he's also fairly absent as a father, given the nature of his job. Also, since Holden had just suggested that the absence of the father could contribute to becoming a murderer, this could suggest that Tench is wondering if his lack of being an active father figure to his son is potentially shaping him to become a violent person.