I just say "good one Jim" as I woukl to the rest of them.
JLM, I think that would be about appropriate reaction. As to the rest, I can see two arguments. Some probably just want to make fun of him.
However, others may genuinely want to encourage him. If he rolls a strike or a spare only once in a while, it is quite an achievement compared to someone who does it all the time. For him it is a much bigger achievement than it would be for you. So it stands to reason that somebody would think that he deserves more praise than you would in a similar situation.
So it could be considered patronizing behavior. But insulting? Perhaps yes, perhaps no.
Let me give you another example. Suppose a couple has two kids, one gets straight As, the other can only manage Bs and Cs. Now suppose he gets an A. The parents would probably praise him much more than they would the other kid, who may have got 5 or 6 As in the same exam.
Patronizing? Perhaps. But instilling? Hardly.
No one is trying to make fun of him, they just think he'll feel good about himself if they "fall all over him", whereas I know full well if I was Jim, I would resent being treated different as I would take it as recognition of my handicap. JUST MY OPINION THOUGH. With the brothers report cards you mention, maybe one is clever and the other is dumb or maybe one is a hard worker and the other is lazy. In the case of lazy I might tend to play it up, while in the case of dumb I may tend to play it down. Just because it's easier to change lazy than dumb.