Basically, before checking the PKMN for that day, you save your game outside of his mansion (or even inside of his mansion, before you talk to him). After you save, go talk to him, and see what PKMN he says is in the garden - if you don't like it, turn off your game, and turn it back on again, and repeat the process.
It won't work if you talk to him, find out what he's got, save the game, then talk to him again. Unfortunately for me, I didn't think this method would actually work, so I've been waiting for a Porygon for like 3 weeks now, and after trying this method, I got it on the 5th reset... so now I know : / Tomorrow I'll be going for an Eevee.