No one in his family knows about the engagement

I don't think you're overthinking this at all, he definitely is not telling his family for some reason. I would talk to him honestly about what is going on and if he continues to dodge your line of questioning you guys need to have a conversation about continuing the engagement at this time.

My FH proposed once before, on Christmas 2017. He didn't want to tell ANYONE about it. He ended up breaking up with me 2 weeks later. We ended up patching things up and got back together 2 weeks later, but I was a lot more hesitant this time around. When we got engaged this time, It was mandatory that there were people we cared about around to witness it. I ended up talking to his dad and essentially asking for his son's hand in marriage before making it official.

All this is to say that if he's not telling them and he can't give you a good reason WHY I would be very wary. That's a major red flag. You guys definitely seem to have a lot to talk about, because I know from experience how terrible that feeling is. You shouldn't have to keep your engagement a secret from people as important as family (or anyone for that matter).

I'd tell him we are going to dinner with your parents tonight and telling them. There's no reason as to why he hasn't been able to tell them.

This would upset me if FH didn't tell anyone in his family. I agree with PPs that this can be a huge red flag, and an honest discussion is needed to see why he's keeping this a secret. Good luck!