We caught up with Matthew and his brothers, Joey and Andrew, in L.A. this week, where the former child star told us it's unlikely a reboot will happen following Robin Williams' death.

Matthew, who played one of Robin's kids in the original, added ... "Without Robin, I don't think that movie is possible. In 2014, they were talking about it ... and everybody was kind of buzzing about it."

Unfortunately, Robin died by suicide in August of 2014 -- and Matthew says, personally, he just "wouldn't want to do it without him."

Though, Matthew isn't ruling out reuniting with his former 'Doubtfire' costars on another project ... mentioning a documentary about the beloved film could make for an interesting follow-up.

He noted, "Documentaries are pretty cool. So, I don't know, maybe we could do that!"

This update comes after Matthew reunited with his onscreen sister Lisa Jakub ... who revealed on the "Brotherly Love Podcast" how Robin went to bat for her after she was expelled from school while filming "Mrs. Doubtfire."

As Lisa recalled ... Robin wrote a letter to her school principal begging them to reconsider her expulsion, proving the Oscar winner was one of the nicest guys in Hollywood.

Robin had been suffering from an undiagnosed case of Lewy body disease -- a form of dementia -- when he died at the age of 63.

You can still hear what an impact he had on his young 'Doubtfire' costars when they talk about him today.

We discussed Robin's legacy in a previous episode of the Last Days Podcast, available on all podcast platforms.