I don't think he has any moral obligation to donate the amount asked by Lodsys. Putting his head on the block like this is already helping a lot more than $3500 given to lawyers could.
He definitely doesn't have a moral obligation, but it is a fairly minimal number to help prove his intentions are genuine and not simply a strategy to save $3,500.
Also, he should stress the risk he is taking in defending his position since most readers will not have any idea how big it is.
What?! Why would he be obligated or in any way suggested that he donate the sum Lodsys are asking for, isn't that the whole point of this: it's a ridiculous claim and he isn't required to pay the money.