Despite David being understandably freaked by that point, the obvious relief his eight year older parents was completely understandable.
Sixteen year old Jeff is very supportive. That relationship was what made the movie. The way Jeff had completely changed — the way David suddenly had to rely on him, and how that worked. Jeff was so much looking out for David, even signalling him to get away at the end, and it made their reunion at the very end of the film all the more special. For a time, Jeff had been the Big Little Brother, and David hadn't forgotten it, and you could see how that memory had affected him when they reverted to their old roles in the end, and David was kinder to his brother and more willing to trust him, showing him what he'd obtained and everything.
Jeff at 16: Don't take any shit, David!
David and Max, just before the time jump to the past.
David: Let's get outta here.
Max: Compliance, navigator.
David reuniting with his present time family at the end.