Fingers crossed he'll be ok and I think Mischief is a great name!
As kittens grow so quickly, I would wait until he's around 6 months old, neutered and ready to go outside before putting on a collar. Many cat owners do not agree with collars as they can cause terrible injuried if they are caught on branches, furniture etc, so it's really important to get a safety release collar. The one's sold here
www.kittycollars.co.uk are excellent and really good value for money.
When he gets neutered, i'd also get him microchipped so that if he ever wanders off and loses his collar, he's still identified as yours!!!
Good luck with it all - he looks quite little but some of the kittens we get into care look like little scraps and fight their way through to become strong and healthy cats so lots of love and attention and it'll be fine!
Has he been de-fleaed and de-wormed yet?