"don't recall" I'm guessing this is largely just an ass-covering strategy to avoid giving the opposing side an upper hand. If you say "yes" to anything you might give them verbal grounds to convict you of more than they can directly find and show external evidence for.
If he simply doesn't want to talk about something, isn't he entitled to take the 5th and just refuse to answer? Saying he doesn't recall is a gambit because it's an obvious lie but I think he's hoping it's one we're so used to hearing that it sounds less incriminating than refusing to answer.
As in once you waive your right to the 5th and take the stand, you have to answer all of the opposing side's questions truthfully.
(Not a lawyer.)