Try having a house full of eevee do it.
On a serious note, anonymous, a lock on a bathroom door is a gift sent by Ho-oh for anyone raising baby pokémon.
Or for a more serious answer, eventually, it’s possible that your snivy will outgrow this tendency. In the meantime, try to give him a comfortable environment while at the same time training him to get used to moments without you. There are tips located here that may help: http://bills-pokedex.tumblr.com/search/separation
Overall, don’t forget that your snivy is still an infant, so it’s understandable that he will be a little clingy. It’s important for now that you try not to train him to expect contact with you at all times while balancing his needs as a baby pokémon.
Best of luck, anonymous!