ok heres my theory Axel does remember Ven but knows 100% that Roxas is Sora's nobody so he just throws it off but if you seen the opening to 358/2 days Axel asks Roxas "are you really sure that you don't have a heart" im guessing that axel is remembering Ven and thinks Roxas is Ventus

well everything is possible in KH expect the unexpected

He thinks Roxas is Ventus? No, you see by this time. It was common knowledge in the Organization that Sora and thus Roxas had a connection to Ven. Wether or not the members knew that Ven's heart existed inside of Roxas. (Possible, concerning how Xemnas spoke to Roxas, while he was unconscious). I just think he didn't mention it to him, because it would sound weird and it's probably something the game designers or the translators or something overlooked.

They still could have had him vaguely allude to Ven in the secret reports, but they didn't, and it ended up being the worst post game secrets to unlock to date.

Axel met Ventus once for 10 years ago, chances are that he doesn't remember him at all. Even when he met Roxas he didn't remember him. Axel doesn't ask Roxas about Ventus because he doesn't remember him.

Mickey met Xehanort, the person who would continually try to destroy everything he knows and love in the same timeframe, and continued to actively fight against him for years, but only just remembered his name when he saw Xemnas' face in Hollow Bastion.

I think Axel has a perfectly legitimate excuse to forget compared to that.