If one eats pizza and is 100% certain he did not say the bracha for it, may he still say it? Explanation: Eating a piece of pizza, realizing you didn’t say the bracha, and saying the bracha before eating another piece of pizza.
Yes, if he hasn't finished his meal, he should say the bracha before continuing to eat. Sulchan Aruch Orach Chaim 167:8:
If one forgot himself and began to eat without having made the blessing HaMotzi, then if he realised this during the meal he should make the blessing then.
Even if he's not hungry anymore, he should eat a little more so that he can say the bracha.
This doesn't apply if he isn't sure whether he said it or not.