I agree with Amenhotep II's answer. At that age, I and all the boys I knew just thought that girls were weird. Of course, in perfect kid logic, that mostly didn't apply to our sisters. Our attitudes changed massively once we became teenagers. So don't take your boy's attitudes at 5 too seriously. I think you're doing all that's necessary for now - you're telling him girls can do everything boys can do.
I'd just like to point out something you might think about:
Each time he indicates a gender bias, we convey to him that girls are as strong as boys and can do everything that boys can do
I realize your son is 5 and at that age, your statement is obviously true. But it won't stay true - a large percentage of boys will grow stronger than girls in the same age cohort in early puberty. He might already realize that this is going to happen sometime in the future because he's seen TV programs or comics where gender roles are clearly defined, especially in respect to strength. Think about how many superheroes are depicted as female, for example, or how many football/soccer games with mixed teams you see on TV. Females actually can't compete with males in several sports.
So maybe telling him that girls can certainly do everything that boys can do, but he's right that boys tend to become stronger once they're older might be the better course of action. You might accompany this with some thoughts about how strength really isn't a very important property of a person, unless the person is living about ten thousand years ago and hunts saber tooths and mammoths...
Acknowledging physical differences between genders doesn't strike me as a problem. You're not denying that some people have darker skin color than others, either,or that some have black hair while others are blond, or that some people don't grow as tall as others, or that some people have a genetic tendency to grow heavy while others stay skinny. The important thing would be to make sure he understands that yes, girls might be less likely to win a push-up competition against boys, but that doesn't imply that they're less capable in any other respect, just as a heavy person might not win a race against a skinny one, but it doesn't tell you anything about what that person is capable of in other areas.
I'm tempted to add that you could tell him that while boys tend to grow stronger than girls, boys are more likely to die than girls starting from conception, a trend which continues to old age, which is probably why slightly more boys are conceived than girls. But I realize this is a fairly stupid thing to tell a 5-year-old boy.