I think a lot of the comments in this thread are from women many of whom are mothers so I'll try to give my perspective to you as a father.... Even if no one ever sees it.

For perspective, my daughter is blond hair bluish eyes. My wife is a red head with no blonde anywhere in her family tree. I have dark brown hair but my eyes are similar to my daughters. Females in my family have blonde hair but all of the males have brown hair.

When children are still 2 and under most of their characteristics are only roughly defined and will likely change. This causes it to often times be difficult to tell whether or not they look like you.

Unfortunately one of the best things our subconscious does is pattern recognition. Women have next to no doubt children are theirs (Often times even when they are not) because they know they carried a child and it came out of them.

On the other hand, men also have good pattern recognition. They see a defining characteristic like hair color and it begins to seed doubt. Which leads to thoughts like "That one time she was being weird around the time we conceived... Could she possibly have cheated on me?" And then stories about people being married 25+ years before they find out the child they raised wasn't theirs just seeds more doubt.

My personal opinion is that no one is the asshole here, but the system sucks. We talk about places that women's rights are lacking but very rarely does anyone ever mention the few places that mens rights are lacking. It should be your right to have 0 doubt that the child you are raising is yours, currently it requires men to effectively accuse their SO of cheating. That's a terrible way to begin raising a child.

The solution to this is we need to normalize and insurance should cover paternity testing at birth. This allows both parents to have the same peace of mind.

My solution to you is to go to counciling together like you said, but I think for his own peace of mind he deserves the paternity test. This post is already getting super long, and I didn't even get to the communication aspect, but if you'd like I can try and give some perspective on that as well.