Spoiler/reflection/theory one: Jimmy is obviously Lester's dad, and is not a brain damaged as he seems at first.
For starters, half of the dialogue in the comic only makes sense if Jimmy is Lester's dad, especially the poignant look after "You and me get along, and you never had any kids." However, the second part only comes into play with that in mind. My thought is that the hit Jimmy took in the NHL was only the last in a series of concussions, which made it dangerous to continue playing professionally, and left him with speech problems and concentration problems. Those of you familiar with football, think Steve Young in recent years. Therefore, instead of a man with the mind of a child playing with a child, it's a man who can't get respect in his home town connecting with his son the only way he's allowed.
Spoiler/reflection/theory two: Arguments for and against alien invasion.
On the first hand, if there are no actual aliens, what the hell happens to Jimmy? Lester may be imaginative, and writing Jimmy out of the story may have been dramatic, but Lester lives his story and wouldn't kill off a real person like that for no reason. This makes even more sense when you consider the Lester hung up his cape, meaning something important happened. Also, he seems as surprised as anyone when he flies or did the trick with the flaming slingshot pellet.
On the other hand, this is a story about people connecting with one another, or failing to, as the case my be, and that means alien invasion is probably out. Also, Lester never uses his powers when anyone other than Jimmy is around.
Seems kind of like a crap shoot to me, so believe whatever you prefer.