I don't know why they do it but he's not alone. It could be a stress reliever I'd be guessing really. If I remember I'll ask my colleague who deals with cat behaviour stuff!
I'd be keen to hear anything about this as our two have always done it. It seems to be an obsession as they actively hunt out carrier bags from any hiding place we find!
All of our cats have done/do that. They don't eat the plastic or bite bits off, they just chew; and seem to enjoy it. Seems a not dissimilar pastime to humans popping the bubbles in bubble-wrap.
My cat Pegerty LOVED plastic bags. If you came in to the house she would practically throw herself at you and try and shove her head in. The little bugger would sit & rustle them as a way to wake me up too