Pumpkin is a lovely cat, and he seems to have mastered the cat nap! There are many good articles and friendly and knowledgeable folks here. If you can't find what you need, feel free to ask!
Pumpkin is quite the handsome kittie and looks so sweet sleeping there. I'm sorry he's having such a hard time getting used to being 100% indoors. You're absolutely right in getting him a new cat tree and some toys - preventing him from being bored will help him make the transition. Give him lots of play time with you, too, perhaps with a wand toy or a laser pointer. Poor boy, this can be tough on an old guy set in his ways but he really is better off inside and you're doing the right thing. Just keep telling yourself that when he's pleading with you! 
