It's not cruel so long as you take the commitment to keep him occupied . Mine are now all indoor cats and they are very happy that way. Get him a cat tree [Pet Supplies, Pet Food, and Pet Products on Sale Now at zooplus.co.uk, or ebay have reasonably priced ones] get him some toys [you don't need to spend a fortune, some good cat nip ones and balls of paper and empty boxes are always well received and you can keep changing them to stop him getting bored if you don't spend too much . Fishing rod toys are fab]
and the company he gets from you and your family will keep him busy.

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