I haven't seen this film.
In "Summer Lovers" a woman (Darryl Hannah- a real-life Aspie) is faced with a similar predicament as yours. She's hurt and angry, but also finds the other woman to be a nice person. She befriends her, and then brings her into their orbit as a threesome and it works- for a time. Like I said, not for everyone.
As Ste11eres alludes to, how far would you/should you go to keep him? It does sound like he's not into monogamy. That's one word I'd address directly to him to break that "silence" of his.