HI, your poor lad has a lot to adjust to, and will be missing his garden, so you need to make his indoor home environment as stimulating as possible. I've got two young cats who don't have free outdoor access yet (long story), so we have to find ways to let them burn off energy and keep entertained so they don't wreck the house!
Give him places to climb, eg
http://www.amazon.co.uk/Scratching-Heig ... 292&sr=1-5
Make places where he can play and climb and watch the world go by.
Also lots of toys that he can play with on his own (it might take you a while to work out what he likes, but get some good high strength catnip mice, and my lot love soft foam rubber balls). When you're with him, make plenty of time to play with him. Da Bird is very good, also laser pens, and throw toys, so he gets plenty of energetic exercise.
I also put an extra water bowl out for them, and sometimes an extra food bowl, away from their normal feeding mat, just to give them a chance to seek and find.
This is a start. Maybe others will come along with more tips for you. Thanks for caring about your boy's welfare. You''ll have as much entertainment as him keeping him happy and occupied!