Ohr hayashar to hilkhot mamrim 6:3 points this out, but doesn't seem to answer.

Daf Al Hadaf to Pesahim 56a quotes several answers. The sefer merafsin igra (page 233) is cited as suggesting that in reality the sons added honorifics to his name that are omitted by the verse.

Alternatively, it suggests on the basis of berakhot 4a that an inherently flattering name may be used without appellations. The name Yisrael is flattering, alluding to his overcoming various forces.

Ohr HaYashar to Hilkhot Mamrim (6:3) points this out, but doesn't seem to answer.

Daf Al Hadaf to Pesahim (56a) quotes several answers: the Sefer Merafsin Igra (page 233) is cited as suggesting that in reality the sons added honorifics to his name that are omitted by the verse.

Alternatively, it suggests on the basis of Berakhot (4a) that an inherently flattering name may be used without appellations. The name Yisrael is flattering, alluding to his overcoming various forces.

Like the first explanation, This commentator (I can't read the name from this resolution. Will update from a hard copy) explains that they reffered to "our father Yisrael". He supports this with Targum Pseudo-Jonathan's wording:

וַאֲמָרוּ לֵיהּ שְׁמַע יִשְׂרָאֵל אָבוּנָן יְיָ אֱ-לָהָנָא

They said to him: Listen Yisrael, our father, God is our Lord...

See also Sdei Hemed (Vol. III: Kaf: Klal 104; pg. 122).