Tomahawk has some very good points about fighting cheese w/ cheese. As others have noted however, Tau have so many options to deal with rhinos and razorbacks, it's actually more difficult to narrow down your options.
My own inclination in dealing with this would be at least one Optimized Stealth Cadre for vehicle killing. Supplement this with Crisis, Broadsides, Ionheads, Riptides... whatever you want. I'd also recommend Piranhas. They are relatively cheap and while you can certainly use them with fusion blasters and/or seekers, I think their main benefit against hoards of razorbacks are their ability to block and channel your opponent's vehicles to force them to make tough choices. The more metal boxes they have, the more their own numbers will begin to work against them. Block easy avenues to your units with some cheap Piranhas and target key razorbacks in his formation in order to use their wrecks to your advantage. Between terrain, Piranhas, and wrecked Razorbacks, it shouldn't be all that difficult to lock him into his own deployment zone.
From there, you can use your Riptides, crisis, overcharged Ionheads or whatever other anti-marine weapons you have available to pound those 5-man squads into the dirt.
It's not an auto-win but you've definitely got options.