Coolio's Son -- Slide, Slide, Slippity Sliding ... to Prison

Coolio's son is about to go on the exact opposite of a fantastic voyage -- he's headed to prison for at least 3.5 years ... TMZ has learned.

As we first reported, 22-year-old Grtis Ivey was arrested on November 14, 2011 for allegedly busting into a Las Vegas apartment with a gun and forcing the tenant into the bathroom while his female prostitute friend ransacked the place.

Law enforcement sources tell TMZ that Ivey agreed to a deal earlier this week in which he agreed to plead guilty to a felony robbery charge ... and in exchange, prosecutors would drop the remaining four felony charges.

Under the plea agreement, Ivey will serve a minimum of 3.5 years in prison (and a max of 10 years depending on how much time the judge believes he deserves).