Yell at him loudly as you are playing. Shout demeaning things like, "Thanks for stealing my drum solo" or "Hey, where's the fire! You're speeding up. Slow down!"
Beat on a cowbell loud enough to be heard over the guitar solo. Keep the beat with him at first and then slow down and make him follow.
Stop playing your instruments. Everybody in the band should stop playing, and when the lead guitarist opens his eyes, continue to stare at him. Get his attention and chastise him by saying, "You're doing it again." Then roll your eyes and shake your head in disappointment.
Walk in front of him, bobbing your head in a very exaggerated manner with the music. When you get his attention, mouth the words "slow down" in an exaggerated manner.
Play back a tape of the guitarist that includes passages in which he is speeding up. Point out to him what is happening, and talk to him about his playing.