As long as you have slowly brought him up in temperature then start off as Meyer says with a little food ,
This I would suggest should be wheatgerm and garlic , he wil then l be eating the same as your other koi when they finally come out of winter into early to late spring when your ponds temperature is above that of 10c for a number of weeks,
As Meyer suggests it is vitally important you match both indoor tankand outdoor pond temperatures before release into the pond.
So let's focus now on the health of your koi indoors seeing as to how this koi is now grounded so to speak we have to think about his accomodation over the next few months.
Please how big is your koi and how big is the tank you have him in ?
Do you have filtration on your tank and is the tank fully mature, if the answer is no it isnt then keep an eye on the water perameters until the tank has matured.
Do regular water changes whilst he is in the tank and we cant see you having a problem , but be aware that he may develope problems quite rapidly as the temperature in the tank rises to match that of your indoor temperature.
I would suggest you do this slowly over a 36 hour period .
Dave