DF questioning paternity

This may be a long post so bare with me...

I've been with my DF for 9 yrs. We have 2 kids and are very happy. Many years ago I f*cked up and took some drugs and ended up having a threesome with a guy and a girl. There was no penatrative sex with the guy. Fast forward afew years, my DF finds out and was extremely hurt but forgave me. It took a long time and alot of work but we got to a place where he could trust me again and we were happy again. It happened before I was pregnant with my first. My second was planned and conceived after DF found out and after we had worked things out. My first has always looked like her father, everyone always tells me that. She has his height, hair colour, eye colour and many of his features and personality. Of late though I've noticed him bringing up the fact that he doesn't think she looks like him at all. He keeps saying she has none of his features. I'm worried he thinks she's not his. I know for 100% certain that she is and I've told him there was no penatrative sex when I cheated but I'm not sure he believes me. I keep telling him that she does look exactly like him (except a female version) but I'm not sure if I should bring up the past again and say again that there wasn't any penetration or if I should leave that in the past and just keep reminding him of her traits that make her look like him. I would with no hesitation do a DNA test if he asks but I don't want to be the one to suggest it. I just don't know what to do. He says he trusts me and believes what I've told him but I think a part of him is questioning whether our daughter is actually his.

Has anyone been in this situation? Any advice?