My partner likes to play smite and adds new friends regularly which is fine, however he recently added a person who has admitted they like men and on their profile states they are a man-eater (this isn't the issue, just gives context). (More context, he is diagnosed autistic) Recently I found out from my partner this person had said to him things like "ttyl ❤️" and "goodnight ❤️". He didn't send any hearts back (he had already told them before this he was in a relationship) and didn't realise what could be implied here so I told him to ask them why they sent that. They got upset/angry asking if he was implying that they like him and they don't go after men in a relationship so I let him know that it's not appropriate for them to send messages like that to someone who has a monogamous relationship and mentioned he should let them know to stop. He said he did this but I found out today after pressing the matter he did not. In fact he mentioned that I saw the messages and wanted to know about it, the person replied that 'that's crazy 😱" to which he mentioned it's what happens when I have had toxic relationships in the past. Now I feel that he cannot be trusted as he did not do what he said he did and also possibly encouraged this person by not properly defending me, as despite this person calling me crazy for being uncomfortable with them sending hearts, he still continues normal friendly conversation.
I want to know if I am being paranoid, if his odd behaviour around this is possibly due to the autism and how can I trust him again after this?
I am willing to consider couples therapy, although not sure how to go about it in the UK.
Thanks for any help
[Edit] Just to add more context, this isn't the first time he has hidden conversations with women from me, including his ex, although nothing actually happened in those conversations. My main concern is that he keeps hiding things from me, despite me explaining that I don't mind him having female friends, I just have boundaries about flirting etc. And explaining that the hiding things makes it harder to believe there is no intent to do something that could harm our relationship. We have been together about three years, but live some distance so see each other usually at weekends if that helps at all. It doesn't help that he previously mentioned he might struggle in bed with me as I am larger than what he sees online but that conversation was in the first few months of our relationship, and we actually discovered since then it was due to medication he is taking and potentially some mental health concerns.