I agree with most of the posts here. When Frodo finds out about Boromir's death, there must be many thoughts inside his head. He was being interrogated by the brother of the Man who tried to seize the One Ring from him. After being tormented by the Ring for months, coming to know Boromir's dead, his own cousins and all. He needed to be strong very much.
And I agree with those who said Fro cannot hold grudges against anyone, and especially Boromir. Cuz now he himself has realized the evil power of the Ring.
Well, he forgave Gollum, and Saruman so it is obvious from his part to forgive Boromir.