I'm very sorry to hear about this! Poor little Shadow!

When all the kittens would play, he'd just be purring away on my lap or neck
This sounds cute but I can't help but wonder why he wasn't playing with his siblings. Maybe, like someone suggested, a genetic problem or a heart disease. It's sad, but it happens.

For the short time Shadow was here on earth, he was loved and cared for, and I'm sure he knows that. But I know, this is no comfort for you at the moment because it's just too sad...

Kirsten