As a rule of thumb, you should wait until the risk of night frosts has passed (a bit like putting your non-hardy garden plants out). If it feels too cold for you to stand outside with no coat on, it'll be too cold for your bunnies too. However, if he's been in an unheated room then you can probably put him out around April time, as long as it's really starting to feel like Spring and we're not having hard frosts etc.