If you absolutely have to put him back outdoors then you must wait until the temperature has risen and will stay steady and only go up so i.e. until all risk of frost/ice has passed and spring is definitely here. Usually in the UK that's about May. In the past we have had snow and very low temperatures in April, so don't be tempted to put him out too early as his coat won't be thick enough and the shock of a sudden unexpected low overnight temperature could very well kill him.