You didn't ask, but I'll offer: FACE TO FACE. For me, this wouldn't even be a decision. Make it hard on him to look you in the eye and explain the company's and his position on this.
Also, in the States, just bear in mind that the savings ISN'T 90%/10%; there are costs to employees that don't just disappear when they go (some of the taxes, the sunk costs of the employee so far though thankfully we're early in the year if your fiscal year is a calendar year, unemployment if he's eligible). Still ask for your 25%, but make a commitment that you can do it without hiring a new person; they may not be right, but they MIGHT be looking at this like "I'm giving this Kade dude a permanent 10% raise for holding our nutsacks for a couple of weeks until we replace that dude that bailed". Make the case why it's more than that.
And good luck! If you're doing the job of two, you ought to be paid for it!