For the most part, if you're playing as Janna, you want to try and facilitate his farming as best you can. This might mean walking up with him when he farms(something not enough supports do honestly; just you being next to the ADC while they farm is added pressure; hiding behind your AD or just AFK'ing in the brush is basically allowing them to 1v2 the lane since they don't have the needed pressure to safely walk up; bot lane is a lot of zones of control and you want your "zone" to over lap with your ADCs at least a little) or harassing the enemy support/ADC safely to make them focused on you and what you're doing rather than him and what he's doing.
At a point, he might want to roam and you should try and follow him up with that roam; Twitch roams well for an ADC thanks to his stealth.